DUBAI - A suicide bomber killed 21 worshipers on Friday in a packed Shi'ite mosque in eastern Saudi Arabia, residents and the health minister said, the first attack in the kingdom to be claimed by Islamic State fighters.It was one of the deadliest assaults in recent years in the largest Gulf Arab country, 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 bloodied people moaning with pain as they lay 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," worshiper Kamal Jaafar Hassan told Reuters by phone from the scene.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. It said it would not rest until Shi'ites, which the group views as heretics, were driven from the Arabian peninsula.Saudi officials have said the group is trying hard to attack the kingdom, which as the world's top oil exporter, birthplace of Islam and champion of conservative Sunni doctrine, represents an important ally for Western countries battling Islamic State and a symbolic target for the militant group itself. In November the Sunni group's leader Abu Bakr al-Baghdadi called for attacks against the Sunni rulers of Saudi Arabia, which has declared Islamic State a terrorist organization, joined international air strikes against it, and mobilized top clergy to denounce it.Last week Baghdadi issued another speech laden with derogatory comments about the Saudi leadership and the country's Shi'ite minority.Friday's bombing 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.