Pakistan said on Wednesday an "unprovoked and cowardly" air strike by US forces had killed 11 Pakistani soldiers on its border with Afghanistan and undermined the basis of security co-operation.

The soldiers were killed at a border post in the Mohmand region, opposite Afghanistan's Kunar province, late on Tuesday as US coalition forces in Afghanistan battled militants attacking from Pakistan, a Pakistani security official said.

The US military said in a statement issued on Wednesday that it had co-ordinated the artillery and air strike with Pakistan, but was investigating further. The incident came as frustration is rising in Kabul and 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. In its strongest criticism of the US military since joining the US-led campaign against terrorism, the Pakistani military condemned the killing of the 11 paramilitary soldiers, including an officer.

If confirmed, it would be the most Pakistani soldiers ever killed in an attack by US forces. The attack "hit at the very basis of co-operation and sacrifice with which Pakistani soldiers are supporting the coalition in the war against terror," the military said.

"Such acts of aggression do not serve the common cause of fighting terrorism," it said in a statement.

Prime Minister Yousaf Raza Gilani also condemned the attack. "We will take a stand for sovereignty, integrity and self-respect and we will not allow our soil (to be attacked)," he told parliament.

Earlier, a Pakistani security official said the soldiers were killed after militants had launched an attack into Afghanistan. "The militants launched a cross-border attack into Afghanistan and our soldiers were killed in a counter-offensive by forces in Afghanistan," said the official, who declined to be identified.

In response, the US military said the counter-offensive had been aimed at anti-Afghan militants and Pakistan had been told in advance - part of an operation "had been previously co-ordinated with Pakistan."

"Shortly after the attack began, coalition forces informed the Pakistan army that they were being engaged by anti-Afghan forces in a wooded area near the Gorparai checkpoint," the statement said.

"At that same time, an unmanned aerial system also identified anti-Afghan forces firing at coalition forces. In self-defense, coalition forces fired artillery rounds at the militants."

In Washington, Pentagon spokesman Bryan Whitman said the incident took place in a border area that has long been a shared US-Pakistani concern. "Because we know that the border region is used as a seam by the enemy, we have any number of co-ordination measures and mechanisms in place for working with the Pakistani military because we are sensitive to that," he told reporters.

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

The militant spokesman, Maulvi Omar, said by telephone he had heard that US aircraft had also bombed a nearby Pakistani post, while the Taliban had captured seven Afghan troops and shot down a helicopter.

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

A new Pakistani government has been negotiating with elders of 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.

The government, which came to power after supporters of staunch US ally President Pervez Musharraf were defeated in a February election, is led by the party of former Prime Ministe