ISLAMABAD: Pakistan said on Sunday it was reviewing its alliance with the United States and Nato after up to 26 soldiers were killed in cross-border Nato air strikes, plunging frosty US ties into deeper crisis.

Pakistan sealed its Afghan border to Nato, shutting down a lifeline for the estimated 130,000 US-led foreign troops fighting the Taliban, and called on the United States to leave a secretive air base (Shamsi) reportedly used by CIA drones.

Islamabad protested to Nato and the United States in the strongest terms -- summoning US ambassador Cameron Munter, branding the strike a violation of international law and warning that there could be serious repercussions.

The US-led Nato force in Afghanistan admitted it was “highly likely” that the force's aircraft caused the deaths before dawn on Saturday, 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.

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 John Allen, the US commander in Afghanistan, discussed coordination with Pakistan's army chief General Ashfaq Kayani.

Pakistan said Nato helicopters and fighter aircraft fired “unprovoked” overnight Friday-Saturday 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”.

In Afghanistan, Allen promised a thorough investigation “to determine the facts” and extended his condolences to the loved ones of anyone who died.

Munter expressed “regret” over any loss of life and pledged the United States would work “closely” with Pakistan to investigate.

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 each side accusing the other of not doing enough 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.

Americans have long accused Pakistan of playing a double game with the Taliban, and the issue came to a head in September when the then top US military officer, Admiral Mike Mullen, accused Pakistan of colluding in a US embassy siege in Kabul.

US drones carry out routine missile attacks on Taliban and al Qaeda operatives in Pakistan's lawless tribal belt, where American officials say neutralising militants is vital to winning the war in Afghanistan.

Pakistan last week forced its envoy to the United States, Husain Haqqani, to step down over accusations that he sought American help in limiting Pakistan's powerful military after the bin Laden raid.

His successor, Sherry Rehman, has yet to arrive in Washington.