Pakistan has blocked a vital supply route for international forces in Afghanistan in apparent retaliation for an alleged cross-border helicopter raid by Nato troops that killed three Pakistani frontier soldiers.

Over the weekend, Nato helicopters fired on targets in Pakistan at least two times, killing several suspected fighters they allegedly pursued over the border from Afghanistan.

Pakistan's government protested against the attacks, which came in a month during which there have been an unprecedented number of drone missile attacks in the country's northwest.

Pakistan also threatened to stop providing protection to Nato convoys if the military alliance's helicopters attacked targeted inside Pakistan again.

Kamal Hyder, Al Jazeera's correspondent in Islamabad, said on Thursday that there are reports of 100 Nato supply lorries already being held up at the Torkham border post in retaliation for the earlier cross-border Nato raids.

"A Nato convoy had been moving through Khyber Pass, but the trucks were turned back from entering the Khyber region," he said. "We are also told that the CIA chief [Leon Panetta] in Islamabad has been addressing this issue."