The United States moved to salvage ties with Pakistan after NATO air strikes on border outposts killed up to 26 soldiers and plunged relations into crisis, backing a full probe and expressing condolences.

Pakistan said it was reviewing its alliance with the US and NATO after the lethal strike which it branded a violation of international law, summoning the US ambassador Cameron Munter and warning of serious repercussions.

It also announced the closure of its Afghan border to NATO, shutting down a supplies lifeline for 130,000 US-led foreign troops fighting the Taliban, and called on the US to leave a remote air base reportedly used by CIA drones.

The NATO force in Afghanistan admitted it was "highly likely" that its aircraft caused the deaths in Saturday's pre-dawn strike, inflaming US-Pakistani relations still reeling from the May killing of Osama bin Laden.

The US commander in Afghanistan promised a full investigation and sent his condolences over any troops "who may have been killed" on the Afghan border with Pakistan's lawless tribal belt, branded an Al-Qaeda hub by Washington.

US Secretary of Defense Leon Panetta and Secretary of State Hillary Clinton bolstered that message in a joint statement Sunday, offering their "deepest condolences" and backing "NATO's intention to investigate immediately."

Clinton, the US commander in Afghanistan General John Allen, and the chairman of the US Joint Chiefs of Staff, General Martin Dempsey, called their Pakistani counterparts to discuss the situation.

"These US diplomatic and military leaders each stressed -- in addition to their sympathies and a commitment to review the circumstances of the incident -- the importance of the US-Pakistani partnership, which serves the mutual interests of our people," the US statement said.

"All these leaders pledged to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time."

NATO troops frequently carry out operations against Taliban insurgents close to the border with Pakistan, which in many places is unmarked, although the extent to which those operations are coordinated with Pakistan is unclear.

Afghan and US officials accuse Pakistani troops at worst of colluding with the Taliban or at best of standing by while insurgents fire across the border from Pakistani soil, often in clear sight of Pakistani border posts.

At the same time Pakistan, battling its own Taliban insurgency in the northwest and dependent on billions of dollars in US aid, gives the US-led war effort in Afghanistan vital logistics support.

Key questions remain unanswered about what exactly happened in Mohmand district, just hours after General Allen discussed coordination with Pakistan's army chief General Ashfaq Kayani.

Pakistan said NATO helicopters and fighter aircraft fired "unprovoked" on two army border posts, killing 24 to 26 troops and wounding 13, adding that Pakistani troops had returned fire.

The government said the attacks were "a grave infringement" of sovereignty, a "serious transgression of the oft-conveyed red lines".

A spokesman for NATO's International Security Assistance Force (ISAF), Brigadier General Carsten Jacobson, confirmed that foreign soldiers, working with Afghan troops, called in air support for an operation near the border.

"It's highly likely that this close air support, called by the ground forces, caused the casualties," Jacobson told AFP.

Pakistan swiftly sealed its border with Afghanistan to NATO supplies -- holding up convoys at the Torkham and Chaman crossings on the main overland US supply line into landlocked Afghanistan from the Arabian Sea port of Karachi.

An extraordinary meeting of cabinet ministers and military chiefs ordered the United States to leave the Shamsi air base within 15 days, despite reports that American personnel had already left.

It also said the government would "undertake a complete review of all programmes, activities and cooperative arrangements with US/NATO/ISAF, including diplomatic, political, military and intelligence".

The Pakistani military said funerals will be held at 9:30 am (0430 GMT) Sunday in the northwestern city of Peshawar for those soldiers killed.

Relations between Pakistan and the United States have been in crisis since American troops killed bin Laden near the capital without prior warning and after a CIA contractor killed two Pakistanis in Lahore in January.

Pakistani, US and Afghan officials have traded complaints about responsibility for cross-border attacks, with accusations that not enough is being done to prevent insurgent assaults on military positions.

In September 2010, Pakistan shut the main land route for NATO supplies at Torkham for 11 days after accusing NATO of killing three Pakistani troops.

The border was reopened after the United States formally apologised.

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