Nato helicopters 'kill Pakistan checkpoint soldiers'
The "unprovoked and indiscriminate" attack took place in the Pakistani tribal region of Mohmand, the Pakistani military said in a statement, BBC News reports.
In response, Pakistan has closed the border crossing for supplies bound for Nato forces in Afghanistan.
Nato said it was aware of "an incident" near the border and was investigating.
The alleged attack took place at the Salala checkpoint, about 1.5 miles (2.5 km) from the Afghan border, Reuters reports, at around 02:00 local time (21:00 GMT).
Two officers were among the dead, officials said, and seven soldiers were reported wounded.
Prime Minister Yousuf Raza Gilani "has strongly condemned the Nato/Isaf (International Security Assistance Force) attack on the Pakistani post," the foreign ministry announced according to AFP news agency.
"On his directions, the matter in being taken up by the foreign ministry, in the strongest terms, with Nato and the US," it added.
The BBC's Shoaib Hasan in Karachi says Pakistani officials are furious, arguing there was no militant activity in the area at the time.
The incident risks dealing a fresh blow to US-Pakistan relations, which had only just begun to recover following a unilateral US raid that killed Osama Bin Laden in Pakistan in May.
Supplies halted
A senior Pakistani military officer told Reuters news agency that efforts were under way to transport the bodies of the dead soldiers to Mohmand's main town of Ghalanai.
"The latest attack by Nato forces on our post will have serious repercussions as they without any reasons attacked on our post and killed soldiers asleep," he said, requesting anonymity because he was not authorised to talk to the media.
Masood Kausar, governor of Khyber Pakhtunkhwa province, condemned the attack in a statement, reported AFP news agency.
"Such cross-border attacks are unacceptable and intolerable," he said, adding that the government would take up the matter at the highest level and launch a thorough investigation.
In apparent response to the attack, lorries and fuel tankers were being stopped at Jamrud town in the Khyber tribal region near the city of Peshawar, officials and local media said - part of a key supply route which delivers 80% of Nato's equipment to Afghanistan.
"We have halted the supplies and some 40 tankers and trucks have been returned from the check post in Jamrud," Mutahir Zeb, a senior government official, told Reuters.
Pakistani troops are involved in fighting the Taliban in the crucial border region area. Hundreds of militants have been resisting attempts by the security forces to clear them from southern and south-eastern parts of the district.
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