The United States defended itself against charges its forces killed 11 Pakistani troops in an incident on Wednesday that threatened to worsen already tense ties with its key war-on-terror ally.

The US-led coalition in Afghanistan admitted carrying out an air strike in Pakistan on Wednesday but said it targeted militants - not the Pakistani soldiers - and that its response was coordinated with Pakistan.

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 the border region.

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.

Bruce Riedel, a senior analyst at the Brookings Institute as well as former CIA officer and former top advisor to three US presidents on South Asian affairs, worried about the political fallout.

"The timing is terrible," Riedel told AFP.

"Just when the internal struggle in Pakistan to resolve (President Pervez) Musharraf's fate is coming to a head and our strained relations with the new rulers are very tense, we get a major incident on the border killing Pakistani soldiers," he added.

"Whatever little pressure Pakistan has been putting on Al-Qaeda is likely to get even smaller," he said.

Rick Barton, an analyst with the Centre for Strategic and International Studies, echoed his remarks.

"The new government needs to be encouraged," he said. "When the US makes mistakes of this sort, it produces pressure, and it will produce instability."

In Washington, Pentagon spokesman Geoff Morrell tried to appease Pakistan with which he said there exists "extremely important" ties but he defended the US military's actions and refused to confirm Pakistani military deaths.

"Although it is early, every indication we have is that it was a legitimate strike in self-defence against forces that had attacked the coalition forces," Morrell said.

Gonzalo Gallegos, a State Department spokesman, said the United States is "sad to see the reported loss of Pakistani life. However, our troops were defending themselves against a hostile act, which they have the right to do."

He said he wanted to clarify his earlier remarks in which he said "we're sad to see the loss of life among the Pakistani military, who are our partners in fighting terror."

Gallegos added that the US ambassador to Islamabad, Anne Patterson, met with Pakistan's foreign secretary Salman Bashir to "discuss the incident," but gave no details of the talks.

According to the Associated Press of Pakistan, Bashir conveyed the "resentment from the government to the ambassador."

Bashir told Patterson that "Pakistan strongly condemned the air attack on the Frontier Corps' border checkpost at Gora Prai in Mohmand Agency by the Coalition Forces based in Afghanistan," it said.

"The attack was unprovoked and a gross violation of the international border between Pakistan and Afghanistan," he added.

"The senseless use of airpower against a Pakistani border post by Coalition Forces is totally unacceptable," he said.

"It constitutes a blatant and wilful negation of the huge sacrifices that Pakistan has made in its endeavour to combat terrorism."

State run television quoted Senate leader Raza Rabbani as saying that foreign minister Shah Mehmood Qureshi will raise the issue during a meeting in Paris with Rice.