The Islamic State (IS) group says it was behind a suicide bombing on a Shia mosque in Saudi Arabia that killed at least 21 people.
The attack in Saudi Arabia's Eastern Province is the first to be claimed by the Saudi branch of IS, which was formally established last November.
The claim was posted on Twitter with an image of the bomber by an account that is a reliable source on the group.
IS has previously threatened to attack Shia Muslims in Saudi Arabia.
Witnesses reported a huge blast at the Imam Ali mosque in the village of al-Qadeeh, in Qatif governorate during Friday prayers.
The Saudi health minister told state television that at least 21 people had been killed and more than 80 injured in the blast.
A doctor at the hospital said that some people had been wounded "very critically".
Television pictures of the bombing showed shattered glass and debris inside the mosque, where scores were said to have been praying.
A spokesman for the Saudi interior ministry said the bomber detonated a suicide belt inside the mosque, causing a number of people to be "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.
Analysis: Frank Gardner, BBC security correspondent
It will come as no surprise that the Islamic State group was behind this calculated attack on peaceful worshippers in a Saudi mosque.
The Sunni extremists of IS have a special loathing for the Shia and they will almost certainly have been looking to exacerbate sectarian tensions between Saudi Arabia's Shia minority and the rest of the population.
Shia anger will likely be directed at the Saudi authorities for failing to protect their community in Eastern Province.
Shias have been concerned for some time at inflammatory speeches given by hardline Sunnis that depict them as being 'not real Muslims', a view shared by IS.
In a statement published online, IS said "the soldiers of the Caliphate" were behind the attack and pledged "dark days ahead" for the Shia community. It identified the bomber as Abu Amer al-Najdi.
In a separate statement, the jihadist group also said it was behind a similar attack on a Shia mosque in neighbouring Yemen on Friday, which injured about a dozen people.
Saudi Arabia has previously been threatened by IS, which is being targeted by a US-led coalition in Syria and Iraq.
In November last year, an audio recording purported to be from IS chief Abu Bakr al-Baghdadi called on Saudi followers to attack Shia targets.
More than 2,000 Saudis are believed to have joined IS, with hundreds having returned home. Saudi Arabia has launched a security crackdown in recent months, arresting hundreds of IS suspects.
Saudi Arabia's large Shia Muslim minority is mainly based in the Eastern Province, and there have been sporadic protests by Shia there for greater rights.
The attack on Friday comes at a time when the Saudis are leading a coalition of Arab states in an air campaign against Shia rebels in Yemen.