Islamic State, the Sunni Muslim dominated militant group, claimed responsibility on Tuesday for an assault on a Shi’ite Muslim mosque in Oman that left at least nine people dead, including three attackers, a rare security breach in the oil-producing Gulf state.

Four Pakistanis, an Indian and a police officer were among those killed in the gun attack, according to Pakistani, Indian and Omani officials. The Oman police said 28 people of various nationalities were wounded, including security personnel.

The attack began on Monday evening at the Ali bin Abi Talib mosque in the Wadi al-Kabir neighbourhood of Oman’s capital Muscat, authorities said, 500 metres from an international school and an adjacent skateboard park and less than 10 kilometres from a string of five-star beach resorts.

Such violence is exceptional in the wealthy, Sunni Muslim-dominated Gulf states – ordinarily secure and stable – raising fears that Islamic State, which has operated in the shadows since it was largely crushed by a U.S.-led coalition in 2017, may be attempting a comeback in new territory.

Islamic State said in a statement late on Tuesday that three of its “suicide attackers” fired on worshippers at the mosque on Monday evening and exchanged gunfire with Omani security forces until morning.

The group also published what it said was a video of the attack on its Telegram site.

Another video shared on social media and verified by Reuters showed people running from the mosque while gunfire could be heard.

Police have not said whether they have identified a motive for the attack or made any arrests. Omani authorities also have not released the identity of the attackers.

Read more: Foreign Office confirms four Pakistanis died in Oman attack

A local source said the mosque was also known as Imam Ali mosque and is a Shi’ite place of worship in Ibadi-ruled Oman, which has a small but influential Shi’ite minority.

Describing the incident as a “terrorist” attack, the Pakistani foreign ministry said 30 survivors were being treated in hospitals.

‘Unprecedented’

Islamic State said its fighters attacked a gathering of Shi’ite Muslims who were “practicing their annual rituals.”

Monday evening marked the beginning of Ashura, an annual period of mourning, which many Shi’ite Muslims mark publicly, to commemorate the 7th century death of Hussein, a grandson of the Prophet Mohammad. The observation of Ashura has sometimes triggered sectarian tensions between Sunni and Shi’ite Muslims in some Middle East countries, though not typically in Oman where the Ibadi sect promotes tolerance.

Most Omanis adhere to Sunni Islam or to the Ibadi faith, which is a branch of Islam that has much in common with mainstream Sunni Islam.

“This is a very unprecedented event … the likes of it we have not seen in Oman’s history,” Pakistan’s ambassador to Muscat, Imran Ali, said after visiting some victims in hospital.

He said most of the 30 victims there were being treated for gunshot wounds while others had suffered injuries fleeing the attack, including being crushed in a stampede.

In March, the Islamic State group said it was behind an attack that killed more than 140 people at a concert hall near Moscow, and in January it claimed responsibility for two explosions in Iran that killed nearly 100.

Such high profile attacks have stoked fears of a comeback for a group with clandestine leadership and whose fighters are thought to be scattered in autonomous cells.

At its height of its power in the early-2010s, Islamic State declared a “caliphate” over a wide area of Syria and Iraq, imposing death and torture on detractors, and inspiring attacks in dozens of cities around the world.

The group’s control collapsed after a sustained military campaign by a U.S.-led coalition.