PESHAWAR, Pakistan (AFP) - - Pakistan said a "cowardly" air strike by US-led forces killed 11 Pakistani troops on Wednesday near the Afghan border and warned that it had harmed cooperation in the war against terrorism.

The army accused the US-led coalition in Afghanistan of launching an unprovoked attack on a checkpost in Pakistan's volatile Mohmand tribal zone while the foreign office demanded an investigation.

In Kabul, the coalition admitted carrying out an air and artillery strike in Pakistan but said it was targeting militants hiding near the paramilitary outpost and that it had informed Pakistani forces.

The incident, the worst of its kind since Pakistan joined the "war on terror" in 2001, comes amid growing unease in Washington and Kabul over Pakistan's efforts to negotiate with Taliban militants.

In an unusually harsh statement, a Pakistani army spokesman "condemned this completely unprovoked and cowardly act" and said 11 soldiers died in the overnight air strike, including an officer.

"The incident had hit at the very basis of cooperation and sacrifice with which Pakistani soldiers are supporting the coalition in the war against terror," the statement quoted the spokesman as saying.

Prime Minister Yousuf Raza Gilani condemned the incident, telling parliament: "We will take a stand to preserve the sovereignty, dignity and respect of the country."

President Pervez Musharraf backed the US-led toppling of Afghanistan's Taliban regime after the 9/11 attacks on the US, but his support for Washington has angered many Pakistanis and attracted the wrath of militants at home.

The foreign office condemned the "senseless use of air power" by the coalition, adding that the "attack also tends to undermine the very basis of our cooperation with the coalition forces and warrants a serious rethink on their part of the consequences that could ensue from such rash acts."

Islamabad later summonsed US ambassador Anne Patterson to lodge a protest.

The official Associated Press of Pakistan reported that Foreign Secretary Salman Bashir conveyed the "resentment from the government to the ambassador".

Pakistani security officials said the deaths came after Afghan troops crossed the porous frontier and tried to occupy the strategic Pakistani post in the troubled tribal belt, which borders eastern Afghanistan's Kunar province.

The post was in an area long disputed between the countries.

Pakistani troops repulsed the Afghan soldiers and the coalition then bombed the area. Coalition aircraft also killed around 15 Taliban about a kilometre (half a mile) away, the officials said.

Heavily armed Pakistani tribesmen with rocket launchers and Kalashnikov rifles gathered near the checkpost in the mountainous Gora Prai area to show their support after the attack, residents said.

The dead soldiers' bodies had been sent to their hometowns for burial, state media said.

The US-led coalition said an investigation was ongoing but did not specifically refer to the Pakistani allegations about the deaths.

In a statement, it said its soldiers had repelled a militant attack during an operation in Afghanistan that was previously coordinated with Pakistan.

Coalition forces informed the Pakistani army that they were coming under fire from "anti-Afghan" forces in a wooded area near the Gora Prai checkpoint in Pakistan, it said.

Unmanned drone aircraft identified the militants and "in self defence" the coalition fired artillery rounds and then used close-air support "until the threat was eliminated."

No coalition troops crossed the border, it said.

"We always reserve the right of self defence in these matters," a Pentagon spokesman, Bryan Whitman, said in Washington.

Later the US State Department called the deaths "regrettable". Spokesman Gonzalo Gallegos added: "We're sad to see the loss of life among the Pakistani military, who are our partners in fighting terror."

"This is a reminder that better cross-border communications between forces is vital," the director of press relations said, reading from a statement.

A spokesman for Pakistani Taliban militants, Maulvi Omar, said eight "mujahedeen (holy warriors)" were killed by coalition helicopters.

Pakistan has protested over a series of missile strikes attributed to US-led forces in Afghanistan in recent months.

Several Pakistani soldiers have also been killed by stray shells, but it appears to be the first time any have been killed by a targeted air strike by US forces.

The attack came two days after a think tank funded by the US Department of Defence said members of Pakistan's intelligence services and its paramilitaries were supporting the Taliban.