The fault lines between Ukraine and Russia run deep. Western and Ukrainian officials have raised alarms about a Russian military buildup near Ukraine, where Russian-backed separatists have been fighting Ukraine's Western-allied forces since 2014.
The Russian moves have raised concern over the possibility of the conflict escalating or even a Russian invasion of Ukraine -- accusations that Moscow rejects.
The animosity was stoked Friday when Ukrainian President Volodymyr Zelensky asserted that a group of Russians and Ukrainians had plotted a coup attempt in Ukraine. A Kremlin spokesperson denied any Russian role in the alleged plot.
• Why is there tension between Russia and Ukraine?
In March 2014, Russia annexed Crimea from Ukraine. A month later, war started between Russian-allied separatists and Ukraine's military in the eastern Ukrainian region of Donbas. The United Nations human rights office estimates that more than 13,000 people have been killed.
Ukraine and the West say Russia supported Ukraine separatists with troops and weapons, but Moscow has said any Russian individual participation was voluntary.
Various peace efforts have fallen through.
• Why are there concerns now?
Ukraine has repeatedly decried the growing Russian troop presence near its borders, saying the mobilization is an attempt to pressure Ukraine. Russia says the forces have only taken part in military exercises, but has strongly opposed any possible moves for Ukraine to join NATO or forge closer strategic ties to the West.
Ukraine has responded with military exercises at its borders.
Moscow has dismissed the accusations of a possible Russian invasion, calling the Western warnings a smear campaign. Kremlin officials have said Russia moving its troops within its own territory should not be a concern to others.
• What are other nations saying?
Ukrainian Prime Minister Denys Shmyhal, in an interview with Politico, urged NATO to send warships to the Black Sea and reconnaissance flights to monitor Russia. Polish President Andrzej Duda also called for a stepped-up allied military response to Russia's military build-up.
The United States has signaled its concerns about a bolstered Russian troop presence near the Ukrainian border. The Kremlin has called out the West for failing to encourage compliance with peace deals from Ukraine, as well as providing Ukraine with support and holding joint drills.
• What is the U.S. doing?
In an April phone call with Zelensky, President Joe Biden pledged "unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression."
Since 2014, the United States has supported Ukraine financially and militarily, giving over $4.5 billion in assistance, including two shipments of Javelin antitank missiles from the Trump administration to Ukraine.
During a meeting this month with the Ukrainian foreign minister, Secretary of State Antony Blinken expressed concerns about reported unusual Russian military activity and stressed the U.S. commitment to Ukraine's sovereignty.
"Our concern is that Russia may make a serious mistake of attempting to rehash what it undertook back in 2014, when it amassed forces along the border, crossed into sovereign Ukrainian territory and did so claiming falsely that it was provoked," Blinken said. "So the playbook that we've seen in the past was to claim some provocation as a rationale for doing what it intended and planned to do."