In response, Pakistan has closed the crossing, blocking NATO supplies from entering Afghanistan.  The incident is expected to further complicate an already fraught relationship between Pakistan and the United States.
In a statement, the NATO-led International Security Assistance Force said that it was investigating and expressed its condolences to the families of those killed.
“My most sincere and personal heartfelt condolences go out to the families and loved ones of any members of Pakistan Security Forces who may have been killed or injured,” General John R. Allen, commander of the NATO-led forces, said in the statement.
The Pakistan prime minister, Yusuf Raza Gilani, “strongly condemned the NATO/I.S.A.F. attack,” according to a statement released by the Pakistani Ministry of Foreign Affairs.
Barrister Masood Kausar, the governor of northwestern Kyber-Pakhtunkhwa Province, called the attack “unprovoked and highly condemnable” while talking to AAJ TV, a private news network.
“This incident is highly regrettable and condemnable. We think there is no justification,” Mr. Kausar said. “This is not a small incident. It is being taken very seriously.”
The incident took place overnight at a military post in Salala, a village near the border with Afghanistan’s Kunar Province. At least 40 soldiers were deployed at the post, which according to Pakistani officials was established to repulse cross-border attacks by Afghan militants and the Taliban.
The Associated Press, citing two local officials, reported that the checkpoint had been recently set up in Salala by the army to stop Pakistani Taliban militants in Afghanistan from crossing the border and staging attacks.
Mehmood Shah, a retired brigadier and analyst based in Peshawar, said the matter should be taken to the United Nations Security Council. Mr. Shah said Americans wanted to make Pakistan a scapegoat after facing failure in Afghanistan.
The border crossing closed at Torkham runs through the Khyber Pass and is the main crossing to Afghanistan from Pakistan. It is used by NATO to ship supplies into Afghanistan.
The incident also comes just a little more than a year after coalition helicopters killed three Pakistani security guards in a series of strikes. Pakistan responded by temporarily closing the border crossing at Torkham.
The border incident comes a day after Pakistani Army Gen. Ashfaq Pervez Kayani met Gen. Allen Jones, the commander of coalition forces in Afghanistan, in Rawalpindi. The two generals had “discussed measures concerning coordination, communication and procedures between the Pakistan Army, I.S.A.F. and Afghan Army, aimed at enhancing border control on both sides,” according to a statement by the Pakistani military.
The Afghan-Pakistan border has been a source of tension between the two countries, as they and the United States trading accusations of inadequate security to prevent cross-border attacks.
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