The supply route through Pakistan, which passes through dangerous regions, is critical for international forces
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."
Our correspondent also said that the Pakistani government is making quite a big noise about it. Pakistan said that Isaf [the Nato-led force] and Nato must respect the mandate under which they are operating. They are expressing deep concerns that, despite the fact that [Isaf] knew where these positions were, they still went ahead."
Critical supply route
Khyber is on the main Nato supply route through Pakistan into Afghanistan, where more than 152,000 US international troops are fighting the Taliban.
This supply route is critical for non-military supplies for Isaf: it is reported that up to 250 vehicles a day cross the Pakistan border into Afghanistan as part of the Nato supply chain. Pakistan's relations with Nato are already strained over the intensifying drone attacks in the border regions.
Rehman Malik, the Pakistani interior minister, said of the border incident: "We will have to see whether we are allies or enemies."
A permanent stoppage of supply vehicles would place massive strains on Nato and hurt the Afghan war effort.
Speaking from Kabul, Sue Turton, Al Jazeera's correspondent, said on Thursday that Nato had launched an investigation into the reports.
"Initial reports said that [the Isaf helicopters] didn't encroach on Pakistani air space. Nato claims it carried out the attack on the Afghan side of the border. If they do want to cross the border, which coalition forces say they often do in self-defence, they usually get in touch with their Pakistani counterparts beforehand, if not during the operation," she said.
Nato has said previously that it has the right to self-defence. The multinational force has on at least one other occasion acknowledged mistakenly killing Pakistani security forces stationed close to the border.
The surge in suspected Taliban activity inside Afghanistan and apparent increased willingness by Nato to attack targets on the border, or just inside Pakistan, could be a sign the international forces are losing patience with Pakistan.
The country has long been accused of harbouring Afghan Taliban fighters in its lawless tribal regions.