Saudi authorities say they believe the bomber set off an explosives belt hidden under his clothes once he was inside the Imam Ali mosque in eastern Saudi Arabia, during busy Friday prayers.

It's believed more than 150 people were in the building at the time of the explosion.

A video posted online showed victims covered in blood lying in a smoke-filled hall.

The Saudi health minister told state television more than 90 people were wounded, although numbers remain unclear.

State media reported the Saudi Interior Ministry saying the assault was carried out by "agents of sedition trying to target the kingdom's national fabric," and promising that "security authorities will spare no effort in the pursuit of all those involved in this terrorist crime."

Top Saudi cleric, Grand Mufti Abdel-Aziz al-Sheikh told state television the attack is an attempt at "driving a wedge among the sons of the nation," calling it "a great sin."

The extremist group "Islamic State" (IS) claimed responsibility for the attack, the first in the Saudi kingdom to be officially claimed by the group.

In a statement published online, the terror organization identified Abu Ammar al-Najdi as the bomber, and called the Shiite worshippers "heretics."

Saudi officials say the nation is an attractive and symbolic target for IS, being the world's top oil exporter as well as being aligned with the West.

In November the group's leader, Abu Bakr al-Baghdadi, called for attacks against Saudi Arabia's Sunni rules.

In Washington, the White House was quick to condemn the blast, but was skeptical concerning IS' involvement.

Spokesman Josh Earnest said confirmation of who was responsible for the assault was "still under review," but that attacks targeting Muslims were "all too common."

'Fully responsible'

In nearby Lebanon, the Shiite political party Hezbollah joined in criticism of the blast, but argued that Saudi officials also bore blame.

"Hezbollah holds the Saudi authorities fully responsible for this ugly crime, for its embrace and sponsorship for these criminal murderers," a statement read.

The comments appear to allude to accusations from Iran, allied with Lebanon, that Saudi Arabia supports ultra-orthodox Sunni militants in the region. Riyadh has denied the claims.

The attack took place as worshippers were commemorating the birth of revered figure Imam Hussain from the 7th century.

It comes at a time of heightened tensions between Sunnis and Shiites, with Saudi Arabia and Iran on opposite sides of conflicts in Syria, Iraq and Yemen.

In March, Saudi Arabia launched a military campaign against Shiite Houthi rebels in Yemen, angering Iran. The eastern Saudi region is seen as a Shiite stronghold, considered a minority in the ultra-conservative Arab state.

Residents in the area have claimed they are being discriminated against over their faith, a claim the Saudi leadership rejects.

In November last year assailants with unconfirmed links to IS killed several members of the Shiite community also in the country's east.

On Wednesday, the Syrian Observatory for Human Rights reported IS had taken control of the ancient city of Palmyra, just days after the fall of the Iraqi city of Ramadi.

an/rc (dpa, AFP, AP, Reuters)