The Islamic State is asserting responsibility for a suicide attack that killed more than 20 people in a Shiite mosque in Saudi Arabia. (Reuters)

The Islamic State said Friday that it was behind a blast that killed or wounded scores of worshipers at a Shiite mosque in Saudi Arabia, marking the first time the militant group has claimed an attack in the oil-rich kingdom and raising fears of an expanding sectarian conflict in the region.

There was no immediate comment from Saudi authorities on the Islamic State’s assertion of responsibility, which was carried in written and audio statements distributed by accounts linked with the Islamic State on Twitter.

The Islamic State communique said that a “martyrdom-seeking brother” set off an explosive belt during a gathering of “impure” worshippers, according to the SITE Intelligence group, which monitors militant postings on social media and elsewhere.

The Sunni extremist group views Shiites as Muslim heretics and opposes ties by Saudi Arabia’s Sunni leadership with the West. The same statement called the attack a “unique operation” and referred to the group’s newly declared “Najd Province,” which encompasses central Saudi Arabia and includes the Saudi capital, Riyadh. The Saudi monarchy presides over Islam’s two holiest sites, making the kingdom a hugely symbolic target for Islamist militants.

People protest following a suicide attack on the Shiite Imam Ali mosque in a village in the eastern province of Qatif, Saudi Arabia. (Str/EPA)

In a statement also posted Friday on Twitter, the Saudi Health Ministry said 21 people were killed and 123 wounded in the blast.

The suicide bomber targeted worshipers at a mosque in the village of Qadeeh in the province of Qatif, part of a mostly Shiite enclave about 240 miles northeast of the capital.

An activist, Naseema al-Sada, told the Associated Press that a suicide bomber detonated explosives as worshipers marked the birth of the 7th-century Shiite saint Imam Hussein. The official Saudi News Agency reported an explosion at the mosque but had no further details. The report said authorities launched an investigation into the attack.

Saudi Arabia’s eastern region, which is the heartland of the kingdom’s Shiite minority, has been the scene of sporadic unrest and violence for years. Shiites, who account for an estimated 12 percent of the Saudi population, say they face widespread discrimination from the kingdom’s Sunni leaders. And Shiite protesters have clashed with Saudi security forces during demonstrations for greater rights in the past.

In November, gunmen opened fire on a Shiite religious procession, killing seven people. Saudi officials blamed the attack on militants linked to the Islamic State, a radical al-Qaeda offshoot also known as ISIS or ISIL.

At the time, an audio statement from a person claiming to represent the gunmen praised the Islamic State’s leader, Abu Bakr al-Baghdadi, but did not specify any group linked to the slayings, SITE reported.

A statement from U.N. Secretary General Ban Ki-moon on Friday condemned the attack “in the strongest terms” and said “such attacks on places of worship are abhorrent and intended to promote sectarian conflict.”

A suicide bomber killed 21 worshippers during Friday prayers in the packed Shiite mosque in eastern Saudi Arabia, residents and the health minister said, in an attack claimed by the Islamic State militant group. (Stringer/Reuters)

Also on Friday, Saudi journalist Sukina Meshkhis posted a tweet saying that the Islamic State does “not represent Sunnis.”

“Their filthy hands do not distinguish between Sunni and Shia,” Meshkhis said of Islamic State militants. “May God protect this nation from their evil.”

In March, a Saudi-led military coalition began weeks of airstrikes in neighboring Yemen against Shiite rebels who Saudi officials say are backed by Iran. Tehran has denied it supports Yemen’s Shiite rebels, known as Houthis.

But many Shiite leaders in Saudi Arabia have pledged support for the military campaign in Yemen, where the Houthis practice Shiite traditions different from those in Saudi Arabia and elsewhere.

Security has been increased in the Shiite areas of Saudi Arabia since the air war began.

Murphy reported from Washington. Karen DeYoung in Washington and Heba Habib in Cairo contributed to this report.

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