RIYADH, Saudi Arabia -- A suicide bomber killed at least 21 people Friday in a blast inside a Shiite mosque in eastern Saudi Arabia as worshippers commemorated the seventh-century birth of a revered figure, residents and officials said.
Loyalists of the Islamic State extremist group claimed responsibility for the attack -- the second against Shiites in the kingdom in six months. In November, the Islamic State was accused of being behind the shooting and killing of eight worshippers in the eastern Saudi Arabian village of al-Ahsa.
Despite a string of Islamic State-related attacks over the past several months, which also have targeted police, Friday's suicide bombing appears to be the deadliest in the country in nearly a decade.
At least 21 people were killed and more than 60 wounded, the spokesman for the provincial health services Asad Saoud said, adding that at least 40 were critical cases.
Saudi Arabia's Interior Ministry had not released its count for the number of dead and wounded, but it said a suicide bomber who hid explosives under his clothes was behind the attack. Interior Ministry spokesman Maj. Gen. Mansour al-Turki said the attacker struck the Imam Ali mosque in a village called al-Qudeeh.
In a statement distributed on Twitter feeds linked to Islamic State group loyalists, a group purporting to be the Islamic State's branch in Saudi Arabia claimed responsibility. It could not be independently confirmed whether the new group actually has operational links to the Islamic State, which is based in Syria and Iraq.
The group's statement carried a logo in Arabic referring to itself as the "Najd Province" -- a reference to the historic region that is home to the capital Riyadh and the ruling Al Saud family, as well as the ultraconservative Wahhabi branch of Islam.
Habib Mahmoud, managing editor for the state-linked Al-Sharq newspaper in Qatif, said the attacker stood with the worshippers during prayer and then detonated his suicide vest as people were leaving the mosque.
A local activist, Naseema al-Sada, said the suicide bomber attacked worshippers as they were commemorating the birth of Imam Hussain, a revered figure among Shiites.
The attack happened during a time of heightened Sunni-Shiite tensions in the region, as Saudi Arabia and Iran back opposite sides in conflicts in Syria, Iraq and Yemen.
Just before Saudi Arabia launched airstrikes against Shiite rebels in Yemen in late March, a purported affiliate of the Islamic State claimed responsibility for suicide bombings in Yemen's capital that killed at least 137 people and wounded nearly 360.
The Saudi offensive in Yemen has sharpened anti-Iranian rhetoric inside the kingdom. Saudi Arabia accuses Iran of arming the Yemeni rebels, a claim that both the militias and Tehran deny.
Some ultraconservative Sunnis in Saudi Arabia, known as Wahhabis, have used Friday sermons to rally support for the war and simultaneously criticize Shiites and their practice of praying at the tombs of religious figures, which they view as akin to polytheism.
Mahmoud said people in Qatif were shocked by the attack and "hold those who are inflaming sectarian rhetoric, from those on social media and in the mosques, responsible."
He said that too often the public does not differentiate between what is Iranian government policy and what is Shiite, and "blame Shiites for Iranian actions in the region."
The country's top cleric, Grand Mufti Abdel-Aziz al-Sheikh, told Saudi state television that the attack in Qatif aims at "driving a wedge among the sons of the nation" and described it as "a crime, shame and great sin." The country's top council of clerics issued a statement blaming the attack on "terrorist criminals with foreign agendas."
Residents in the country's eastern region say they are discriminated against because of their faith. They said that despite the region being home to most of the kingdom's oil reserves, their streets, buildings, hospitals, schools and infrastructure are neglected and in poor condition. They say unemployment runs high among Shiite youth in the area.
In 2011, Shiites in the east inspired by the Arab Spring uprising in neighboring Bahrain took to the streets to demand greater rights. Police arrested hundreds of people and a counterterrorism court sentenced an outspoken cleric, Nimr al-Nimr, to death.
After the bombing, a few hundred people marched in mourning through the village, Mahmoud said.
Al-Sada, the activist, said she too holds the government responsible for not doing more to criminalize sectarian rhetoric.
"The government should protect us, not encourage sermons and schoolbooks to incite against us as nonbelievers," al-Sada said. "We want them to prevent this from happening in the first place."
"Martyrdom does not scare us, but we want to live like other citizens and with stability," she said.
Information for this article was contributed by Zeina Karam and Maggie Michael of The Associated Press.
A Section on 05/23/2015