The jihadist Islamic State group said in an online statement on Friday that it carried out a deadly suicide bomb attack at a mosque in Saudi Arabia's predominantly Shiite Eastern Province.
The 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 Shiite-majority city of Qatif.
It identified the bomber as Abu Amer al-Najdi, and published a picture of him.
He struck during the main weekly prayers in Eastern Province, where assailants linked to the Sunni extremist IS killed seven members of the minority Shiite community in November.
The IS statement pledged "dark days ahead" for Shiites until militants "chase them from the Arabian Peninsula".
"It has been established that an individual detonated a bomb he was wearing under his clothes during Friday prayers at Ali Ibn Abi Taleb mosque in Kudeih in Qatif province," the Saudi interior ministry spokesman earlier said.
The spokesman, in a statement carried by the official SPA news agency, said the attack left many dead and wounded, without giving a specific toll.
An activist said earlier that at least four worshippers were killed and others wounded in the bombing, and news websites in eastern Saudi Arabia posted pictures of bodies lying in pools of blood.
Saudi Arabia will "hunt down anyone involved in this terrorist crime carried out by people seeking to undermine national unity," the spokesman added.
Kudeih is located in the Shiite-majority district of Qatif in the kingdom's Eastern Province which has been hit by previous attempts by Sunni extremists to foment sectarian tensions.