Nine persons were killed and as many injured in a missile strike carried out on Monday by pilotless drones on the headquarters of a religious group in Pakistan's troubled Khyber Agency.

At least three missiles struck the headquarters of Amar Bil Maroof Wa Nahi Aneelmunkar (promotion of virtue and prevention of vice force) at Bara in Khyber Agency, where Pakistani security forces launched a crackdown against militants on Saturday.

The missiles apparently targeted Haji Namdar, the head of the religious group, but he was safe, eyewitnesses told TV channels.

The witnesses said six to nine persons had died and several of Namdar's close relatives were among the dead.

Namdar quoted witnesses as saying that missile attack was carried out by drones operated by the US forces in Afghanistan.

The witnesses said they had seen drones flying overhead when the missile strike occurred at about 3 am.

TV channels aired footage of the two-storey brick structure that was almost completely destroyed by the missile strike.

The footage showed rescue workers digging through the debris to remove bodies.

Paramilitary Frontier Corps personnel backed by tanks, artillery and gunship helicopters, launched an operation on Saturday in Bara area of Khyber against three groups -- the Lashkar-e-Islam, Ansar-ul-Islam and Amar Bil Maroof Wa Nahi Aneelmunkar.

The crackdown came in the wake of clashes between members of Lashkar-e-Islam and Ansar-ul-Islam that have claimed over 60 lives so far.

The forces have blown up several militant bases, including the headquarters of Lashkar-e-Islam and Ansar-ul-Islam, though the two groups have offered no resistance to the troops.