Pakistan accused NATO on Saturday 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 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 Saturday.

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.

Saturday'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 southwestern 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 Saturday, 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 United States would work "closely" with Pakistan to investigate the incident.

Relations between Pakistan and the United States 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 in January.

Pakistani, US and Afghan officials have traded increasing complaints about responsibility for cross-border attacks, 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.

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

Americans have long accused Pakistan of playing a double game with Taliban, coming 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.

Pakistan this week forced its envoy to the United States, Husain Haqqani, to step down over a scandal in which he was accused of seeking American help in reining in Pakistan's powerful military after the bin Laden raid.

His successor, liberal rights campaigner and ruling-party lawmaker Sherry Rehman has yet to arrive in Washington.

In Islamabad last month, US Secretary of State Hillary Clinton called on Pakistan to take action within "days and weeks" on dismantling militant havens and encouraging the Taliban into talks to end 10 years of war in Afghanistan.