Pakistan accused NATO on Saturday of killing 26 soldiers in a lethal air strike, protesting in the strongest terms to the US and sealing its border to NATO supplies bound for Afghanistan. It was the deadliest NATO strike 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 \"strongly condemned\" the attack and said \"the matter is being taken up by the foreign ministry, in the strongest terms, with NATO and the US\". Prime Minister Yousuf Raza Gilani cut short a weekend visit to his home town to return to Islamabad for crisis talks with President Asif Ali Zardari, army chief of staff General Ashfaq Kayani, state TV reported. He would hold an extraordinary meeting of senior cabinet ministers and the chiefs of Pakistan\'s army, navy and air force later Saturday, a spokesman said. The military described the pre-dawn attack from NATO helicopters as \"unprovoked\" and \"indiscriminate\" on a sizeable border post in the Baizai area of Mohmand district. The governor of Pakistan\'s northwestern Khyber Pakhtunkhwa province, Masood Kausar, said NATO helicopter fire killed 26 soldiers and wounded 14 others. \"We salute our martyrs,\" he told reporters in Islamabad. 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. General Johan Allen, the US commander of NATO\'s International Security Assistance Force (ISAF), said: \"This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts.\" \"My most sincere and personal heartfelt condolences go out to the families and loved ones of any members of Pakistan security forces who may have been killed or injured,\" he added. 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 cross-border attacks from both sides. 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 ambassador to Pakistan, Cameron Munter, on Saturday expressed \"regret\" over any loss of life and pledged the United States would work \"closely\" with Pakistan to investigate the incident. Pakistan\'s Information Minister Firdous Ashiq Awan said Saturday\'s attack would fan anti-Americanism in the nuclear-armed Muslim country of 167 million. \"NATO attacks breach our sovereignty. Pakistan and its society cannot tolerate this,\" she told reporters. 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. US officials have long accused Pakistan of playing a double game with Afghan Taliban, coming to a head in September when the then top US military officer, Admiral Mike Mullen, accused Pakistan of colluding with the Haqqani faction in a siege on the US embassy 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.