Associated Press: A suicide bomber unleashed a blast in a Shiite mosque in eastern Saudi Arabia as worshippers commemorated the birth of a revered saint, killing at least 21 people and wounding dozens more in the deadliest attack seen in the kingdom in more than a decade. Loyalists of the Islamic State group claimed responsibility for the bombing.

The claim of responsibility, made in a statement circulated on pro-IS Twitter feeds, could not be independently confirmed. It was issued by what purported to be a Saudi branch of the Islamic State group, which is based in Syria and Iraq, but it was not known if the perpetrators had a direct connection with the group's leadership or were sympathizers acting independently in its name.

Still, the bombing highlighted an increasing activeness of IS sympathizers in Saudi Arabia at a time when sectarian tensions have grown over the war in neighboring Yemen, where Saudi Arabia is leading an air campaign against Shiite rebels. Past months have seen a string of smaller attacks on security forces blamed by Saudi officials on the Islamic State group, and in late April, Saudi officials arrested 93 people they said were involved in an IS plot to attack the U.S. Embassy and other targets.

Friday's bombing took place in the village of al-Qudeeh in the eastern Qatif region, the heartland of Saudi Arabia's Shiite Muslim minority, which has long claimed of discrimination in the country, ruled by the ultraconservative Wahhabi interpretation of Sunni Islam.

The bomber stood among the worshippers in the Imam Ali mosque then detonated his explosives as people began to file out, Habib Mahmoud, managing editor for the state-linked Al-Sharq newspaper in Qatif, told The Associated Press. A local activist, Naseema al-Sada, told the AP by telephone from Qatif that the worshippers were commemorating the birth of Imam Hussain, a 7th century figure revered among Shiites.

Al-Manar TV run by the Lebanese Shiite Hezbollah group, carried blurry pictures of pools of blood inside what appeared to be the mosque where the attack took place. It also showed still photos of at least three bodies stretched out on carpets, covered with sheets. One person dressed in a white robe was being carried away on a stretcher.

At least 21 people were killed and more than 60 wounded, the spokesman for the provincial health services Asad Saoud said, according to the state news agency SPA. It appeared the number was likely to rise, with at least 40 critical cases.

That would make it the deadliest militant attack in the kingdom at least since the 2004 attack on residential compounds of foreign workers in the eastern city of Khobar that killed 22, blamed on al-Qaida-linked militants. That earlier attack was part of a wave of al-Qaida-led violence that ended in 2006 as Saudi security forces moved to crush the terror network.

The Islamic State group - formerly al-Qaida's branch in Iraq which broke away and overran much of that country and neighboring Syria - has become notorious for its attacks on Shiites, including a deadly Shiite mosque bombing in the Yemeni capital Sanaa that killed more than 130 people. It was blamed for the killing of eight Shiites in a mosque shooting in eastern Saudi Arabia in November.

The claim of responsibility Friday was issued in the name of a purported IS branch in "Najd Province," a reference to the historic region of a the central Arabian Peninsula where the Saudi capital Riyadh is located.

The attack comes amid heightened Sunni-Shiite tensions in the region as Saudi Arabia and Iran back opposite sides in conflicts in Syria, Iraq and Yemen. 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 "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 say 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 Friday's 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 non-believers," 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.