Pakistan yesterday accused Nato of killing up to 26 soldiers in air strikes, protesting in the strongest terms to the US and sealing its border to Nato supplies bound for Afghanistan.

It was the deadliest Nato attack reported by Pakistan during the 10-year war in Afghanistan

It was the deadliest Nato attack reported by Pakistan during the 10-year war in Afghanistan and looked set to inflame already extremely difficult 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.

Pakistan said the attacks were “a grave infringement” of sovereignty, a “serious transgression of the oft conveyed red lines”, violated international law and “could have serious repercussions” on Pakistan-US-Nato cooperation.

It called in US ambassador Cameron Munter to lodge a strong protest ahead of crisis talks between civilian and military leaders later yesterday.

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.

Yesterday’s incident came just hours after General John Allen, the US commander of Nato’s International Security Assistance Force (ISAF), held talks with Pakistan’s army chief of staff General Ashfaq Kayani on coordination.

The military said Nato helicopters and fighter aircraft fired “unprovoked” overnight on two Pakistan army border posts in Mohmand district, killing 24 troops and wounding 13, to which Pakistani troops responded.

The governor of Pakistan’s northwestern Khyber Pakhtunkhwa province, Masood Kausar, put the death toll at 26 with 14 soldiers wounded.

General Kayani demanded “strong and urgent action” against those responsible, ordering “all necessary steps be under taken for an effective response” to what he called a “blatant and unacceptable act”.

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

“We have stopped Nato supplies after receiving orders from the federal government,” said Mutahir Hussain, a senior local administration official in the tribal district of Khyber.

Pakistani officials at the south-western crossing in Chaman said Nato convoys were also being prevented from crossing.

“This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts,” Allen said yesterday, extending “heartfelt condolences” to the loved ones of anyone who died.

Munter, the US ambassador to Pakistan, expressed “regret” over any loss of life and pledged the US would work “closely” with Pakistan to investigate the incident.

Relations between Pakistan and the US have been in crisis since an American raid killed Osama bin Laden near the capital without prior warning and after a CIA contractor killed two Pakistanis in Lahore last January.

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