KYIV, Ukraine -- Russian President Vladimir Putin claimed victory Thursday in the battle for Mariupol despite an estimated 2,000 Ukrainian fighters still holed up at a giant steel mill, ordering his troops not to storm the stronghold but to seal it off "so that not even a fly comes through."
Western nations, meanwhile, rushed to pour heavy weapons into Ukraine to help it counter the offensive in the east.
U.S. President Joe Biden announced an additional $800 million in military assistance, including heavy artillery, 144,000 rounds of ammunition and drones. But he also warned that the $13.6 billion approved last month by Congress for military and humanitarian aid is "almost exhausted" and more will be needed.
Putin expressed concern for the lives of Russian troops in deciding against sending them in to clear out the sprawling Azovstal plant, where the defenders were hiding in a maze of underground passageways.
Putin's comments came as satellite image provider Maxar Technologies released photos showing more than 200 of what it called new mass graves in a town where Ukrainian officials say the Russians have been burying Mariupol residents killed in the fighting. The imagery shows long rows of graves stretching away from an existing cemetery in the town of Manhush, outside Mariupol.
Mariupol Mayor Vadym Boychenko accused the Russians of "hiding their military crimes" by taking the bodies of civilians from the city and burying them in Manhush.
As many as 9,000 civilians could be buried in mass graves in Manhush, the Mariupol City Council said Thursday in a post on the Telegram messaging app. There was no immediate reaction from the Kremlin.
Boychenko labeled Russian actions in the city as "the new Babi Yar," a reference to the site of multiple Nazi massacres in which nearly 34,000 Ukrainian Jews were killed in 1941.
Maxar said a review of previous images indicates that the new graves were dug in late March and expanded over the past couple of weeks.
After nearly two months of bombardment that largely reduced Mariupol to a smoking ruin, Russian forces appear to control the rest of the strategic southern city, including its vital but now badly damaged port.
But a few thousand Ukrainian troops, by Moscow's estimate, have held out for weeks at the steel plant, despite a pummeling from Russian forces and repeated demands for their surrender. About 1,000 civilians were also trapped there, according to Ukrainian officials.
Instead of sending troops to finish off the defenders in a potentially bloody frontal assault, Russia apparently intends to maintain the siege and wait for the fighters to surrender when they run out of food or ammunition.
Boychenko rejected any notion that Mariupol had fallen into Russian hands.
"The city was, is and remains Ukrainian," he declared. "Today our brave warriors, our heroes, are defending our city."
The capture of Mariupol would represent the Kremlin's biggest victory of the war in Ukraine. It would help Moscow secure more of the coastline, complete a land bridge between Russia and the Crimean Peninsula, which Russia seized in 2014, and free up more forces to join the larger and potentially more consequential battle now underway for Ukraine's eastern industrial heartland, the Donbas.
At a joint appearance with Russian Defense Minister Sergei Shoigu, Putin declared, "The completion of combat work to liberate Mariupol is a success," and he offered congratulations to Shoigu.
Shoigu predicted the steel plant could be taken in three to four days, but Putin said that would be "pointless" and would risk Russian lives.
"There is no need to climb into these catacombs and crawl underground through these industrial facilities," the Russian leader said. "Block off this industrial area so that not even a fly comes through."
The plant covers 4 square miles and is threaded with some 15 miles of tunnels and bunkers.
"The Russian agenda now is not to capture these really difficult places where the Ukrainians can hold out in the urban centers, but to try and capture territory and also to encircle the Ukrainian forces and declare a huge victory," retired British Rear Adm. Chris Parry said.
All told, more than 100,000 people were believed trapped with little or no food, water, heat or medicine in Mariupol, which had a prewar population of about 430,000. Over 20,000 people have been killed in the siege, according to Ukrainian authorities.
Ukraine has repeatedly accused Russia of launching attacks to block civilian evacuations from the city. On Thursday, at least two Russian attacks hit the city of Zaporizhzhia, a way station for people fleeing Mariupol. No one was wounded, the regional governor said.
Elsewhere, Ukrainian Deputy Prime Minister Iryna Vereshchuk said Russian troops kidnapped a local official heading up a humanitarian convoy in the southern Kherson region. She said the Russians offered to free him in exchange for Russian prisoners of war, but she characterized that as unacceptable.
Vereshchuk also said efforts to establish three humanitarian corridors in the Kherson region failed Thursday because Russian troops did not hold their fire.
Russian officials for weeks have said capturing the mostly Russian-speaking Donbas is the war's main objective. Moscow's forces opened the new phase of the fighting this week along a 300-mile front from the northeastern city of Kharkiv to the Azov Sea.
While Russia continued heavy air and artillery attacks in those areas, it did not appear to gain any significant ground over the past few days, according to military analysts, who said Moscow's forces were still ramping up the offensive.
A senior U.S. defense official said the Ukrainians were hindering the Russian effort to push south from Izyum.
Britain's Defense Ministry said that Russia probably wants to demonstrate significant successes ahead of Victory Day on May 9, the proudest day on the Russian calendar, marking the defeat of Nazi Germany in World War II.
"This could affect how quickly and forcefully they attempt to conduct operations in the run-up to this date," the ministry said.
In other developments, Ukrainian President Volodymyr Zelenskyy warned Ukrainians living in areas of southern Ukraine under Russian control not to provide Russians with their IDs, which he said could be used "to falsify a so-called referendum on our land" to create a Moscow-friendly government.
"This is a real possibility," he said in his nightly video address to the nation. "Beware."
NEW U.S. PACKAGE
Biden on Thursday announced a ban on Russian-affiliated ships from U.S. ports and said he would send $800 million more in military resources to help Ukrainian forces -- a package that includes heavy artillery and tactical drones. Biden also announced the United States would provide an additional $500 million in direct economic assistance to the Ukrainian government.
Biden described the help as the "latest steps we're taking to support the people of Ukraine and to hold Putin accountable for his brutal and bloody war."
The latest announcement comes amid what Biden called a "critical window" as Russia prepares for a next phase of war.
Biden met with Ukrainian Prime Minister Denys Shmyhal at the White House, shortly before announcing the additional military aid.
The new $800 million military assistance package for Ukraine represents a marked increase in U.S. shipments of artillery, as well as weapons tailor-made to meet Kyiv's needs on the battlefield.
The package includes 72 155mm howitzers and the tactical vehicles to tow them, and 144,000 additional artillery rounds. That is a significant increase over the 18 howitzers that were in the previous military assistance package, announced earlier this month, and is enough to equip five battalions, said John Kirby, the Pentagon press secretary.
It also includes over 121 Phoenix Ghost Tactical Unmanned Aerial Systems -- a drone that the U.S. Air Force developed "in response, specifically, to Ukrainian requirements," Kirby added. The Phoenix Ghost drones, designed by AEVEX Aerospace, "provide similar capabilities" to Switchblade drones, which were part of a previous U.S. military assistance package to Ukraine.
Pentagon officials would not say whether the new drones were intended to strike vehicles or personnel. According to a senior defense official, Phoenix Ghost is a "tactical" weapon "that is designed to deliver a punch, and you could expect that it would be useful against a number of different targets."
"This is a great example of adapting to their [the Ukrainians'] needs in real time," the official added.
The system "will require some minimal training" for "knowledgeable" drone operators, Kirby said. "We're going to be working through those training requirements directly with the Ukrainian armed forces."
Senior defense officials estimated that the first tranche of weapons from the newly announced package will be dispatched in the next 24 to 48 hours, and that Ukraine will begin to take possession of them by the end of the weekend.
The decision to send the materials "was made in complete consultation and coordination with the Ukrainians about their needs and very much in keeping with their needs specifically in Donbas," Kirby said. But it is not clear how long it will take Ukraine to transfer the weapons to battlefronts in the eastern part of the country.
Senior Pentagon officials declined to weigh in on reports of weapons shipments being stockpiled in the western Ukrainian city of Lviv, with one noting that "we track the deliveries to the border of Ukraine and when the handoff happens ... it's up to them how they move that equipment."
The new assistance package also includes field equipment and spare parts for unspecified weapons systems.
Biden also said that the United States will no longer allow Russian-affiliated ships to enter American ports in his latest update on steps to punish Russia for its invasion of Ukraine.
"That means no ship, no ship that sails under the Russian flag or that is owned or operated by Russian interests will be allowed to dock in the United States port or access our shores," Biden said.
"This is yet another critical step we're taking in concert with our partners in the European Union, the United Kingdom, Canada and further, to deny Russia the benefits of international economic system that they so enjoyed in the past," the president added.
Biden said that it's important that the United States continues to take tougher stances against the Russian government as the war continues.
"As Russia continues to grind out the military advances and their military advances, the brutalities against Ukraine, Putin is banking on us losing interest," the president said. "That's been my view. You heard me say this from the beginning."
"He's betting on Western unity will crack," Biden added. "He's still betting. And once again, we're going to prove him wrong."
The mounting economic damage to Ukraine from Russia's ongoing bombardment has the U.S. and its allies speeding billions in aid to the beleaguered country -- and looking for other sources of cash as well, including Russia itself.
After the U.S. announced $1.3 billion in new economic assistance and military aid to Ukraine on Thursday, Treasury Secretary Janet Yellen acknowledged that "this is only the beginning of what Ukraine will need to rebuild."
The war has already caused more than $60 billion in damage to buildings and infrastructure, World Bank President David Malpass said. And the International Monetary Fund in its latest world economic outlook stated that Ukraine's economy will shrink by 35% this year and next.
The question of who will pay to restore Ukraine from the war has increasingly turned to the Russian state.
Yellen said Thursday that looking to Russia itself for funds to rebuild Ukraine "is something we ought to be pursuing."
When asked about the potential of using frozen Russian Central Bank funds to support Ukraine, Yellen said, "I wouldn't want to do so lightly," adding that it would have to be done in consensus with U.S. allies and partners.
Zelenskyy said in a virtual address to IMF and World Bank leaders that "a special tax on war is needed." He called for the proceeds of sanctioned property and Central Bank reserves to be used to compensate Ukraine for its losses.
He added that frozen Russian assets "have to be used to rebuild Ukraine after the war as well as to pay for the losses caused to other nations."
The Treasury Department imposed a new wave of sanctions against Russia on Wednesday. Included in the sanctions packages are penalties imposed on more than 40 people and entities accused of evading sanctions. The sanctions include the first set of penalties against a cryptocurrency mining firm in relation to the war.
Yellen said Thursday that the U.S. was continuing to "tighten the vise of our economic pressure campaign."
In the continuing war of sanctions and countersanctions, Moscow announced it has barred U.S. Vice President Kamala Harris, tech billionaire Mark Zuckerberg and 27 other prominent Americans, including foreign affairs commentators, from entering Russia.
The move was a response to "ever-widening anti-Russian sanctions" by the Biden administration, the ministry said in a statement, and targeted people it said were shaping a "Russophobic narrative."
Information for this article was contributed by Adam Shreck, Mstyslav Chernov, Felipe Dana, Yesica Fisch, Danica Kirka, Robert Burns, Aamer Madhani, Paul Wiseman, Chris Megerian and staff members of The Associated Press and Paulina Firozi, Ellen Francis, Annabelle Timsit, Adela Suliman and Rachel Pannett of The Washington Post.