A suicide bomber has struck a mosque in south-western Saudi Arabia used by interior ministry's special forces, killing at least 15 people.
The blast in the city of Abha, which is close to the border with war-torn Yemen, is one of the deadliest attacks in the kingdom in recent weeks.
No one immediately claimed responsibility for the attack but the bombing had all the hallmarks of militant groups such as al Qaida or the Islamic State.
An interior ministry statement said 13 were killed and that 10 of those killed were members of the security forces. Security officials later said two more security personnel were confirmed dead, raising the death toll in the attack to 15.
Earlier, the state-owned Al-Ekhbariya news agency gave a higher death toll, reporting that 17 had been killed in the attack.
Interior ministry spokesman Major General Mansour al-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. 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.
There was no immediate claim of responsibility but blame is likely to fall on Islamic State(IS), 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.
Thursday's attack was the deadliest against Saudi security forces since IS attacks first appeared in the kingdom last year.
In May, a suicide bomber struck a Shiite 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, 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 IS 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 IS attacks being plotted in the oil-rich kingdom, including a suicide bomb plot targeting a large mosque in eastern Saudi Arabia that can hold 3,000 worshippers, and attempts to attack other mosques, diplomatic missions and security bodies.
Saudi Arabia is also leading a coalition targeting Iran-allied Shiite rebels in neighbouring Yemen, not far from Abha. The rebels have carried out a number of cross-border attacks against military targets.