IS claims responsibility for Saudi suicide attack that killed 21

More than 150 people were praying when the huge explosion ripped through the Imam Ali mosque in the village of al-Qadeeh, witnesses said.

A suicide bomber killed 21 worshippers during Friday prayers in a packed Shi'ite mosque in eastern Saudi Arabia, residents and the health minister said, in an attack claimed by the Islamic State group.

It was one of the deadliest assaults in recent years in the kingdom, where sectarian tensions have been frayed by nearly two months of Saudi-led air strikes on Shi'ite Houthi rebels in neighboring Yemen.

More than 150 people were praying when the huge explosion ripped through the Imam Ali mosque in the village of al-Qadeeh, witnesses said.

A video posted online showed a hall filled with smoke and dust, with victims laying on the floor littered with concrete and glass. More than 90 people were wounded, the Saudi health minister told state television.

"We were doing the first part of the prayers when we heard the blast," worshipper Kamal Jaafar Hassan told Reuters by phone from the scene.

It was the first attack targeting minority Shi'ites since November when gunmen opened fire during a religious celebration in al-Ahsa, also in the east where most of the group live in predominantly Sunni Muslim Saudi Arabia.

Islamic State said in a statement that one of its suicide bombers identified as Abu 'Ammar al-Najdi carried out the attack using an explosives-laden belt that killed or wounded 250 people, US-based monitoring group SITE said on its Twitter account.

A photograph posted on social media showed the mutilated body of a young man, said to be the bomber.

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The Saudi Interior Ministry described the attack as an act of terrorism and said it was carried out by "agents of sedition trying to target the kingdom's national fabric," according to a statement carried by state news agency SPA.

The agency quoted an Interior Ministry spokesman as saying that the bomber detonated a suicide belt hidden under his clothes inside the mosque.

"Security authorities will spare no effort in the pursuit of all those involved in this terrorist crime," the official said in a statement carried by state news agency SPA.

A hospital official told Reuters by telephone that "around 20 people" were killed in the attack and more than 50 were under treatment at the hospital, some of them suffering from serious injuries. He said that a number of other people had been treated and sent home.

In April, Saudi Arabia said it was on high alert for a possible attacks on oil installations or shopping malls.

In Beirut, Lebanon's Hezbollah, an ally of Saudi Arabia's regional rival Iran, condemned the attack but said authorities in the kingdom itself bore responsibility.

"Hezbollah holds the Saudi authorities fully responsible for this ugly crime, for its embrace and sponsorship for these criminal murderers ... to carry out similar crimes in other Arab and Muslim countries," the Shi'ite group said in a statement.

The statement appeared to echo Iranian accusations that Saudi Arabia sponsors ultra-orthodox Sunni militant groups in the region, an allegation usually taken to refer to groups such as Islamic State and al Qaeda. Riyadh denies the allegations.

In Yemen, a bomb at a Houthi mosque in the capital Sanaa on Friday was also claimed by Islamic State.