A suicide bomber struck a mosque in southwestern Saudi Arabia used by Interior Ministry special forces, killing at least 15 people, ministry officials said.
No one immediately claimed responsibility for the blast on Thursday in the city of Abha, which is close to Saudi Arabia's southern border with war-torn Yemen. But the bombing had all the hallmarks of militant groups such as Al Qaeda or the Islamic State.
An Interior Ministry statement said 13 people were killed, 10 of whom were members of the security forces. Security officials later told the Associated Press that two more security personnel were confirmed dead, raising the death toll in the attack to 15.
Earlier, the state-owned Ekhbariya news gave a higher death toll, reporting that 17 had been killed in the attack. It was not immediately clear why there was a discrepancy, but conflicting reports are common in the chaotic aftermath of bombings.
Interior Ministry spokesman Maj. Gen. Mansour Turki said initial findings point to the attack being carried out by a suicide bomber wearing an explosive vest.
A Saudi Interior Ministry official said the bomb targeted police trainees as they were in the middle of prayer. All officials spoke on condition of anonymity because they were not authorized to speak to reporters. It was not immediately clear if the mosque was inside an Interior Ministry compound.
State media reported that the mosque belongs to an Interior Ministry emergency services post in Abha, which is the provincial capital of Asir. State TV carried images in the aftermath of the attack, which showed blood splattered on the walls and ceiling of the mosque, alongside debris.
Saudi Interior Minister and Crown Prince Mohammed bin Nayef cut short a visit to Egypt, where he was to attend the inauguration of an extension of the Suez Canal, to return to Saudi Arabia following news of the attack, officials said.
Blame for the attack is likely to fall on the Islamic State group, whose local affiliate has claimed responsibility for a number of attacks in recent months, including various deadly shootings and smaller attacks against police at checkpoints in the capital, Riyadh.
In May, a suicide bomber struck a Shiite mosque in the eastern village of Qudeeh, killing 22 people. That was the deadliest militant assault in the kingdom in more than a decade and was followed a week later by another suicide bombing outside another eastern Shiite mosque that left four dead.
Both those attacks were claimed by an affiliate of the Islamic State group fighting in Iraq and Syria and which emerged recently in the kingdom as well. Also, in November, a gunman opened fire at a mosque in the eastern Saudi village of al-Ahsa, killing eight people.
Saudi authorities last month announced the arrest of more than 400 suspects in an anti-terrorism sweep. They said at the time that they had thwarted other Islamic State attacks being plotted in the oil-rich kingdom, including a suicide bomb plot targeting a mosque in eastern Saudi Arabia that can hold 3,000 worshipers.