Pakistan has condemned US-led forces in Afghanistan for an "unprovoked and cowardly" air raid near the border that killed at least 11 Pakistani soldiers.

The Pentagon offered a different interpretation of events on Wednesday, saying US-led forces were acting in self-defence after they had come under fire.

The deaths come at a tense moment in US-Pakistan relations and threaten the already fragile ties.

Kamal Hyder, Al Jazeera's correspondent in Islamabad, said that the US and Pakistan had had a very strong relationship under the previous administration dominated by supporters of Pervez Musharraf, the president.

But Pakistan now had a civilian government, where there is opposition to the alliance in the so-called "war on terror", he said.

Nielson-Green refused to comment on whether it was Pakistani soldiers who were firing on US-led forces but said video footage of the incident was in the process of being declassified.

A statement from the US military said the air and artillery assault was aimed at Taliban fighters nearby and that they had informed Pakistani authorities about the operation.

"Coalition forces informed the Pakistan army that they were being engaged by anti-Afghan forces," the US military said, adding that the operation "had been previously co-ordinated with Pakistan".

Geoff Morrell, a US defence department spokesman, said "every indication we have is that our guys came under attack and they responded".

Affront to sovereignty

In Pakistan, politicians and military officials issued a furious denunciation of the US response as people wounded in the attack were being treated in a Peshawar hospital.

Major-General Athar Abbas, a Pakistan military spokesman, told Al Jazeera there was no provocation from the Pakistani side before the attack.

The US said the Pakistani and American militaries would "look into the matter and review how to prevent a recurrence" and that officials from both sides had already met to "discuss the incident".

The US has given Pakistan $10bn in military aid since 2001, but US officials complain Pakistan has done little to crack down on al-Qaeda or the Taliban in the rugged and largely lawless region along the Afghan border.

The soldiers were killed at a border post in the volatile Mohmand province, a tribal region in Pakistan opposite Afghanistan's Kunar province.

'Returning fire'

The senior US military spokesperson in Afghanistan, Lieutenant-Colonel Rumi Nielson-Green, told Al Jazeera that US-led forces had hit the "right target".

"The people involved in this incident … were firing on coalition forces and they were well inside of Afghanistan when they began the firing," she said.

"This is an absolutely baseless allegation or explanation. We have co-ordination, we have intelligence sharing. If there was some doubt about any post they should have informed us before taking up any strike," he said.

Asked whether Pakistan would continue to co-ordinate with coalition forces, Abbas said: "That is not the point. We have handed over our protest to the coalition forces.

"We will wait for their reply. And whatever is their reply we would like them to consider it seriously."

This was "unprovoked and cowardly … we blame the collation forces … it has hit hard at the basis of our co-operation", he said.

Yousaf Raza Gilani, Pakistan's prime minister, said it was an affront to national sovereignty.

"We will take a stand for sovereignty, integrity and self-respect and we will not allow our soil [to be attacked]," he told parliament.

'Legitimate' strike

The US military in Kabul, Afghanistan, said the operation was aimed at what were described only as "anti-Afghan forces" who had engaged US–led forces inside Afghan territory.

The Pentagon said the strike was "legitimate".

A US military statement said pro-Taliban fighters were sighted in a wooded area near the border post and US-led forces struck with artillery fire.

Maulvi Omar, a Pakistani Taliban spokesman, said that his fighters had attacked US and Afghan forces as they were setting up a position on the Pakistan side of the border.

Eight Taliban fighters were killed and nine others wounded in the subsequent US bombing, he said.

The incident came after Kabul and Western forces in Afghanistan raised doubts about Pakistan's efforts to negotiate pacts with tribal fighters to end violence on its side of the border.

Nato says such deals lead to more violence in Afghanistan.