The Pakistani army said on Thursday its border troops opened fire at helicopters of the International Security Assistance Force (ISAF) after the aircraft violated the country's airspace along the border with Afghanistan.

The incident took place at 3.30 pm (1030 GMT) when two helicopters from Afghanistan crossed into Pakistan territory in the Saidgai Ghulam Khan area of the North Waziristan tribal district, a known sanctuary of al-Qaeda and Taliban militants launching attacks on international forces on the other side of the frontier.

"When the helicopters passed over our border post and were well within Pakistani territory, our security forces fired anticipatory warning shots. The helicopters returned fire and flew back," an army spokesman Major Murad Khan said.

He said the matter has been reported to the ISAF headquarters and the issue is being resolved through existing channels of communication between the sides.

The tensions on the Pakistan-Afghan border have increased in recent weeks as the international forces, particularly those from the United States, step up aerial attacks targeting suspected militant hideouts inside the Pakistani tribal belt.

Stressing that only its troops have the right to take security actions on its soil, Pakistan's army has vowed to defend its territory against any unauthorized trespass.

The latest clash came two days after a US drone crashed in the Pakistani district of South Waziristan. Media reports citing security officials said the drone was in fact shot down by tribesmen and Pakistani troops.