Pakistan lodged a strong protest with the United States over what it called an unprovoked and cowardly air strike by U.S. forces in Afghanistan that killed 11 Pakistani soldiers at a border post.

They died in the Mohmand region, opposite Afghanistan's Kunar province, late on Tuesday as U.S. coalition forces in Afghanistan battled militants attacking from Pakistan, a Pakistani security official said.

The Pentagon defended U.S. forces, saying initial indications pointed to a "legitimate strike" carried out in self-defence after they came under attack.

Frustration is growing among Western forces in Afghanistan over Pakistani efforts to negotiate pacts to end militant violence on its side of the border. NATO says such deals lead to more violence in Afghanistan.

The Pakistani Foreign Ministry said the unprovoked attack was a gross violation of the international border.

"The senseless use of air power against a Pakistani border post by coalition forces is totally unacceptable," it said.

U.S. ambassador Anne Patterson was summoned to the Foreign Ministry over the incident, a government official said.

In its strongest criticism of the U.S. military since joining the U.S.-led campaign against terrorism, the Pakistani military said the killing of the 11 paramilitary Frontier Corps soldiers, including an officer, was "unprovoked and cowardly".

The attack "hit at the very basis of cooperation and sacrifice with which Pakistani soldiers are supporting the coalition in the war against terror", the military said.

Pakistan's ambassador to the United States, Husain Haqqani, told Reuters Pakistan did not view the strike as an intentional hostile act and it was not expected to lead to a Pakistani reconsideration of its U.S. relations.

"LEGITIMATE STRIKE"

Pentagon spokesman Geoff Morrell defended the U.S. troops.

"Every indication we have at this point is that this was indeed a legitimate strike in defence of our forces after they came under attack," Morrell told reporters.

He said the U.S. military greatly valued its relationship with its Pakistani counterpart but U.S. forces were "within their rights to take the action they took".

He declined to rule out the possibility the Pakistani troops could have been involved in the firefight with U.S. forces.

A Pakistani security official said the Pakistani soldiers were killed in a counter-offensive after militants had launched an attack into Afghanistan.

The U.S. military in Afghanistan said it had coordinated the artillery and air strike with Pakistan and it was investigating. The State Department said the incident was regrettable.

A spokesman for the Pakistani Taliban said they attacked U.S. and Afghan forces as they were setting up a position on the Pakistan side of the border and eight Taliban were killed in subsequent U.S. bombing.

Many al Qaeda and Taliban militants took refuge on the Pakistani side of the border after U.S.-led forces ousted the Taliban in Afghanistan in 2001.

A new Pakistani government, led by the party of assassinated former Prime Minister Benazir Bhutto, has been negotiating with ethnic Pashtun tribes to get them to press the militants to give up a campaign of violence in Pakistan in which hundreds of people have been killed over the past year.

Afghanistan and its Western allies say peace pacts in Pakistan's border regions enable militants to regroup and step up cross-border attacks from Pakistani sanctuaries.

Pakistan dropped support for the Taliban after the September 11 attacks on the United States in 2001 but it has been unable to dispel suspicion that elements within its security forces are helping the Taliban, or at least turning a blind eye as the militants organise their insurgency from Pakistan.

Separately, residents of Pakistan's South Waziristan border region said U.S.-led forces in Afghanistan fired into Pakistan on Wednesday but there were no reports of casualties.