Pakistan said on Thursday that its security forces fired at choppers of NATO forces from Afghanistan when they crossed into the Pakistani territory.

"When the helicopters passed over our border post and were well within Pakistan territory, our security forces fired anticipatory warning shots. On this, the helicopters returned fire and flew back," said an army statement.

The statement said the two helicopters from Afghanistan side crossed into the Pakistan territory in the areas of northwestern Pakistan's North Waziristan tribal agency on the Afghan border.

"The matter was reported to ISAF (The International Security Assistance Force) Headquarters and the issue is being resolved through existing coordination and communication channel between Pakistani army and ISAF," said the statement.

But the NATO forces said earlier in a statement that its helicopters were within Afghanistan when they were fired on from a Pakistani military checkpoint, adding that there was no damage or casualties.

"ISAF helicopters received small-arms fire from a Pakistani military checkpoint along the border near Tanai district, Khost while conducting routine operations in Afghanistan," the ISAF statement said.

The coalition forces are increasing their missile and drone attacks on targets inside Pakistan in recent month, blaming Pakistan's local Taliban for rising insurgency in Afghanistan.

The Pakistani side has denounced the cross-border attacks and vowed to open fire if its sovereignty is violated.

(Xinhua News Agency September 26, 2008)