A suicide bomber blew himself up at a Shi'ite mosque in eastern Saudi Arabia during Friday prayers, residents said, and about 20 people were reported killed or wounded.
No one immediately claimed responsibility for the bombing, the first to target Shi'ite Muslims in Saudi Arabia since November when gunmen killed at least eight people in an attack on a Shi'ite religious anniversary celebration, also in the east.
The attack could further harm relations between Sunnis and Shi'ites in the Gulf region, where tensions have risen during weeks of military operations in Yemen by a Saudi-led coalition against Shi'ite Houthi fighters seen as proxies of regional Shi'ite power Iran.
One witness described a huge explosion at the Imam Ali mosque in the village of al-Qadeeh, where more than 150 people were praying.
"We were doing the first part of the prayers when we heard the blast," worshipper Kamal Jaafar Hassan told Reuters by telephone from the scene.
A hospital official said about 20 people had been killed.
State news agency SPA quoted a security spokesman confirming an explosion at a mosque in eastern Saudi Arabia, where most of the country's minority Shi'ites live. A spokesman for the Saudi interior ministry could not immediately be reached for comment.
A photograph posted on social media showed the mutilated body of a young man, said to be the suicide bomber. Other pictures showed ambulances and bloodied victims being taken away on stretchers.
In Yemen, a bomb at a Houthi mosque in the capital Sanaa on Friday was claimed by Islamic State.
