Militant group Islamic State claimed responsibility for a suicide bombing on Monday that killed a worshipper and wounded several others at a Shi'ite mosque in Najan in southern Saudi Arabia.
The attack targeted the "rejectionist Ismailis" said a statement carried by the group posted on Twitter, referring to the Ismaili Shi'ite sect who are a minority in Saudi Arabia.
The kingdom has been hit by a spate of deadly shooting and bomb attacks, many of them laid at the door of Islamic State. The militant group is bitterly opposed to Gulf Arab rulers and is seen as trying to stir up sectarian confrontation on the Arabian peninsula to bring about the overthrow of the ruling dynasties.
It has urged young Saudi Sunnis to attack targets including Shi'ites.
Police found a letter by the bomber addressed to his parents confirming his intention of carrying out the attack, an interior ministry statement said.
The attack on the al-Mashhad mosque occurred as people left after attending evening prayers, residents said.
"As the Magreb prayer ended and the worshippers came out of the main door, a man entered the mosque and blew himself up," Mohammed al-Askar, a doctor from Najran told Reuters, adding that it was the first time a Shi'ite Ismaili mosque had been targeted.
Najran, close to the Yemeni border, is the historic center of the Ismailis, a Shi'ite sect which has long complained of victimization by the prevailing school of Sunni Islam followed by the Saudi state.
In mid-October, five people were killed after a gunman opened fire on a Shi'ite Muslim meeting hall in the Eastern city of Saihat.
In August, a suicide bomber killed at least 15 people in an attack on a mosque used by members of a local security force in southwest Saudi Arabia, an assault that an online statement said was carried out by Islamic State.
Another bombing claimed by Islamic State on a Shi'ite mosque in Kuwait in June killed 27 people.
"Tonight in Najran the hospitals are crowded and people are coming from all over Najran to donate blood," said al-Askar.
"Unfortunately for Saudi I think the Islamic State is going to repeat such treacherous acts. It is a ploy to gain attention. But people are helping each other and they have trust in their government."
(Reporting by Ali Abdelay and Tom Finn; Editing by Richard Balmforth)