The so-called Islamic State group has said it carried out a suicide bombing at a mosque in Saudi Arabia.
A bomber blew himself up at a Shia mosque in eastern Saudi Arabia during Friday prayers, killing around 20 people and wounding more than 50, local residents and hospital officials said.
The IS statement said "the soldiers of the Caliphate" were behind the attack by a suicide bomber "who detonated an explosives belt" in the mosque in the Shia-majority city of Qatif.
It identified the bomber as Abu Amer al-Najdi, and published a picture of him.
The bombing was the first to target Shia Muslims in Saudi Arabia since November when gunmen killed at least eight people in an attack on a Shia religious anniversary celebration, also in the east where most of the country's minority Shias live.
It comes as IS seized the last border crossing between Syria and Iraq controlled by the Syrian government.
The Qatif attack could further harm relations between Sunnis and Shias in the Gulf region, where tensions have risen during weeks of military operations in Yemen by a Saudi-led coalition against Houthi fighters seen as proxies of regional Shia power Iran.
One witness described a huge explosion at the Imam Ali mosque 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 spokesman for the Saudi Interior Ministry, calling the attack an act of terrorism, said the bomber detonated a suicide belt hidden under his clothes inside the mosque, causing a number of people to "martyred or wounded".
"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.
A photograph posted on social media showed the mutilated body of a young man, said to be the bomber. Other pictures showed ambulances and bloodied victims being taken away on stretchers.
