MIRAMSHAH, Aug 21 A pre-dawn drone attack killed at least 21 militants in North Waziristan Agency on Friday.

According to sources, missiles fired by the suspected US pilotless plane hit a residential compound in Dandy Derpakhel village near Miramshah, frequented by militants mostly from Punjab.

Militant sources claimed that women and children, and not their men, had been killed in the attack.

Local people said they heard a loud explosion at about 3.30am. This correspondent heard the familiar sound of drones flying over the area after the explosion.

The compound was adjacent to a large seminary set up by the veteran commander of the anti-Soviet Afghan war, Jalaluddin Haqqani, said to be close to Al Qaeda leader Osama bin Laden.

An official told this correspondent that the compound was used as a training centre for militants but he was not sure which group was running it.

(According to AP, the air strike targeted Siraj Haqqani, a Taliban commander blamed for masterminding ambushes on American troops in Afghanistan, intelligence officials said.

It was unclear if Haqqani, who also had close ties to Al Qaeda, was among the people killed in the attack, the officials said, adding that three women were among the dead. Haqqani is known to have visited the compound.

Friday's strike was the third in three weeks in Pakistan. The US launched the strike based on information that Haqqani was in the compound, according to two of the local intelligence officials based in North Waziristan.

However, Pakistani authorities have not been able to confirm that he was there at the time, they said.

Siraj is the son of senior Taliban leader Jalaluddin Haqqani, who was supported by US and Pakistan when he fought in the 1980s against Soviet troops occupying Afghanistan. Now, American commanders count him as a dangerous foe.)

Soon after the attack, militants attacked the Amin checkpost near Miramshah. An exchange of fire took place which continued for more than three hours.

Security officials, meanwhile, claimed to have killed four militants in a pre-emptive attack but this could not be verified from independent sources.

Political administration imposed a curfew on Miramshah after the attack on the checkpost. The curfew was relaxed for Friday prayers.

Residents said that one person was killed and two others injured when troops opened fire on a group of people for violating the curfew.