ISLAMABAD (AFP) - - The United States insisted Thursday that its forces were retaliating against a "hostile act" when an air strike killed 11 Pakistani soldiers on the murky border with Afghanistan.

Islamabad has accused US-led forces in Afghanistan of making an unprovoked and "cowardly" attack on the checkpost in Pakistan's volatile Mohmand tribal zone, further straining ties between the "war on terror" allies.

In response, Washington said it regretted the "reported loss of Pakistani life" but insisted its forces were targeting militants.

Whatever the circumstances, it is the worst incident of its kind since the Pakistani government sided with the United States in 2001 in its fight against Taliban and Al-Qaeda extremism.

It also comes amid growing unease in Washington and Kabul over Pakistan's efforts to negotiate with Taliban militants.

"The timing is terrible," said Bruce Riedel, a senior Brookings Institute analyst as well as a former CIA officer and top advisor to three US presidents on South Asian affairs.

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

Pakistani security officials say Afghan troops crossed the porous frontier and tried to occupy a strategic Pakistani post in the troubled tribal belt, in an area long disputed between the countries.

The Afghan troops were repulsed, the officials say, after which coalition forces bombed the area, also killing around 15 Taliban nearby.

In an unusually harsh statement, a Pakistani army spokesman "condemned this completely unprovoked and cowardly act" and warned that it had "hit at the very basis" of cooperation in the anti-terror fight.

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

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

The United States is "sad to see the reported loss of Pakistani life," a US State Department spokesman said in Washington.

"However, our troops were defending themselves against a hostile act, which they have the right to do," spokesman Gonzalo Gallegos told AFP.

The Pentagon took a similar line. Spokesman Geoff Morrell said that "every indication we have is that it was a legitimate strike in self-defense against forces that had attacked the coalition forces."

Pakistani President Pervez Musharraf backed the toppling of Afghanistan's Taliban regime after the September 11, 2001 attacks on the United States, but his support has angered many Pakistanis and drawn the wrath of militants.

Earlier this year his party was voted out of office and the new government, led by the former opposition, has sought to strike peace deals with militants in the restive tribal zones bordering Afghanistan.

Insurgents fighting the Afghan government and the NATO- and US-led forces shoring it up regularly cross into the tribal zones, where they find refuge as well as a steady supply of weapons, ammunition and new recruits, experts say.

The US-led coalition said it had informed Pakistan that troops were coming under fire from "anti-Afghan" forces in a wooded area near the checkpoint.

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

No coalition troops crossed the border, it said.

Pakistan has protested over a spate 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 paramilitary forces were supporting the