Riyadh - An alleged new affiliate of the Islamic State group (ISIS) in Saudi Arabia has claimed responsibility for an attack that targeted a mosque used by police special forces in the kingdom's southwest on Thursday, killing 15 people.It was one of the deadliest attacks in years against Saudi security forces that battled a wave of al-Qaeda attacks about a decade ago and which have come under sporadic attacks in recent weeks in the capital, Riyadh, by militants purportedly linked to the Islamic State group.The claim by the so-called Hijaz Province of the Islamic State appeared hours after the attack in the city of Abha, close to Saudi Arabia's southern border with war-torn Yemen. It was carried on IS-affiliated Twitter accounts and was also reported by the SITE Intelligence Group, which tracks militant messages.The statement says an IS suicide bomber targeted a "monument of the apostate". It was the first claim by the purportedly new IS branch, which has not been heard of previously. The "Hijaz" in its name is a reference to the historic western part of Saudi Arabia that is home to Islam's holiest sites of Mecca and Medina.Previous attacks in the kingdom by Islamic State militants have been claimed by a group calling itself Najd Province, a reference to Saudi Arabia's central region, where Riyadh is located.The Interior Ministry said that an initial investigation indicated the suicide bomber wore an explosive belt and struck a mosque inside an Interior Ministry compound in Abha as members of the special forces and trainees were in the middle of afternoon prayers.The ministry's spokesperson, Major General Mansour al-Turki, told The Associated Press it was too early to confirm whether IS or its supporters were behind the attack and that the police were still investigating. Local reports suggested the bomber entered the police compound disguised as a janitor, but al-Turki said this has also not been confirmed.According to state media, the mosque belongs to an Interior Ministry emergency services post in Abha, the provincial capital of Asir. State TV carried images of the aftermath of the attack, showing blood splattered on the mosque walls and ceiling and debris scattered about.The Islamic State's other affiliate in Saudi Arabia claimed responsibility for a number of attacks in the kingdom in recent months, including various deadly shootings and smaller attacks against police at checkpoints in Riyadh.The group claimed it was also behind a suicide bombing in May that struck a Shi'ite mosque in the eastern village of al-Qudeeh, killing 22 people. That was the deadliest militant assault in the kingdom in more than a decade. A week later, a second suicide bombing claimed by the group outside another eastern Shi'ite mosque left four dead.Saudi Arabia is part of a US-led coalition carrying out air strikes against the ISIS in Syria and Iraq. The group has called on supporters in Saudi Arabia to stay and fight the Western-backed monarchy there.In November, police said IS sympathisers were behind the shooting at a Shi'ite place of worship in the eastern Saudi village of al-Ahsa that killed eight people.Saudi authorities last month announced the arrest of more than 400 suspects in an anti-terrorism sweep. In April, they announced the arrest of more than 90 suspects.They said in both sweeps that police had thwarted other IS attacks being plotted in the oil-rich kingdom, including a suicide bombings and attempts to attack diplomatic missions and security bodies.The kingdom is also leading a coalition targeting Iran-allied Shi'ite rebels in neighbouring Yemen, not far from Abha. The rebels have carried out a number of cross-border attacks against military targets.