Pakistan state TV reported that 25 soldiers were killed late last night in an alleged Nato helicopter attack on a Pakistani army checkpoint near the Afghan border.
The Pakistan military has blamed Nato helicopters for the attack on the checkpoint in the Mohmand tribal area and a key Afghan border crossing was later closed to Nato supplies in retaliation for the strike.
A customs official said that he received verbal orders to stop all Nato supplies from moving across the border through the Torkham crossing.
The operator of the border terminal where Nato supply trucks park before getting clearance to cross said the vehicles had been stopped since this morning.
Nato officials in Kabul said they were aware of the incident, and would release more information after they were able to gather more facts about what happened.
The incident came a little over a year after US helicopters accidentally killed two Pakistani soldiers near the border, whom the pilots mistook for insurgents they were pursuing. Pakistan responded by closing a key border crossing on a Nato supply route to Afghanistan for 10 days until the US apologised.
In a statement sent to reporters, the Pakistan military blamed Nato for the attack in the Mohmand tribal area, saying the helicopters "carried out unprovoked and indiscriminate firing". It said casualties have been reported but details were still coming.
The governor of Pakistan's north west Khyber Pakhtunkhwa province criticised the incident, calling it "an attack on Pakistani sovereignty."
The checkpoint that was attacked had been recently set up in Salala village by the army to stop Pakistani Taliban militants holed up in Afghanistan from crossing the border and staging attacks, said two government administrators in Mohmand, Maqsood Hasan and Hamid Khan.
The military has blamed Pakistani Taliban militants and their allies for killing dozens of security forces in such cross-border attacks since the summer.
Pakistan has criticised Afghan and foreign forces for not doing enough to stop the attacks, which it says have originated from the eastern Afghan provinces of Kunar and Nuristan. The US has largely pulled out of these provinces, leaving the militants in effective control of many areas along the border.
The Afghan-Pakistan border is a constant flashpoint, with both nations and the US exchanging accusations of violations and of negligence in preventing cross-border attacks.
The US and Afghan governments have long accused Pakistan of not doing enough to prevent its territory from being used by Afghan Taliban militants and their allies to stage attacks against forces in Afghanistan.
The Afghan government blamed Pakistan for firing hundreds of rockets into eastern Afghanistan earlier this year that killed dozens of people. The Pakistan army has denied it intentionally fired rockets into Afghanistan, but acknowledged that several rounds fired at militants conducting cross-border attacks may have landed over the border.
The Afghan and Pakistani Taliban are allies but have largely focused their attacks on opposite sides of the border. The Afghan Taliban aims to topple the US-allied government in Kabul, and the Pakistani Taliban has tried to do the same in Islamabad.
Frustration about cross-border attacks in both directions has contributed to deteriorating ties between the US and Pakistan. The relationship took an especially hard hit from the covert US commando raid that killed Osama bin Laden in a Pakistani garrison town on May 2.