A hitherto unheard-of affiliate of the Islamic State group claimed responsibility for Thursday’s suicide attack in Saudi Arabia on a mosque frequented by the country’s security forces. At least 15 people, most of them members of the country’s counterterrorism force, were killed in the attack by the so-called Hijaz Province of the Islamic State in the city of Abha, located in the southern province of Asir.

Following the bombing, the group, whose name refers to the historic western part of Saudi Arabia that is home to the Islamic holy cities of Mecca and Medina, reportedly issued a statement claiming responsibility for the attack on the “monument of the apostate.” The ISIS affiliate also vowed to launch more attacks against “tyrants in the Arabian peninsula” who it accused of working for “their crusader masters” -- a reference to Saudi Arabia’s Western allies.

The statement reportedly identified the suicide bomber as Abu Sinan al-Najdi, without providing further details.

Saudi Interior Ministry spokesman, Maj. Gen. Mansour al-Turki, however, said it was too early to confirm whether ISIS was behind the attack, the Associated Press reported.

Saudi Arabia -- a regional Sunni powerhouse -- is part of a U.S.-led anti-ISIS coalition battling the Sunni militant group in Syria and Iraq. The country, which has long been America’s closest ally in the Middle East despite differences on regional policy matters, is also helping the U.S. military train fighters of moderate Syrian rebel groups, according to media reports.

The suicide bombing -- one of the deadliest against Saudi security forces in decades -- is the latest in a series of such incidents in the kingdom. In May, suicide bombers who pledged allegiance to another regional affiliate of ISIS named Najd Province -- a reference to the central Saudi region that includes Riyadh -- claimed responsibility for an attack on a Shiite mosque in eastern Saudi Arabia. This group is also believed to have carried out the attack on a Kuwaiti Shiite mosque in June that killed 26 people.