Nato helicopters attacked a military checkpoint in northwest Pakistan on Saturday, killing up to 28 troops and prompting Pakistan to shut vital supply routes for NATO troops fighting in Afghanistan, Pakistani officials said.
The attack is the worst single incident of its kind since Pakistan uneasily allied itself with Washington in the days immediately following the September 11, 2001 attacks on US targets.
It comes as relations between the United States and Pakistan, its ally in the war on militancy, are already badly strained following the killing of al Qaeda leader Osama bin Laden by U.S. special forces in a secret raid on the Pakistani garrison town of Abbottabad in May.
Pakistan called that raid a flagrant violation of its sovereignty.
The Foreign Office equally condemned Saturday's attack.
''Prime Minister Yusuf Raza Gilani has condemned in the strongest terms the NATO/ISAF attack on the Pakistani post,'' ministry spokeswoman Tehmina Janjua said in a statement. ''On his direction, the matter is being taken (up) by the foreign ministry in the strongest terms with NATO and the US.''
The commander of NATO-led forces in''may have been killed or injured'' during an ''incident'' on the border.
A spokesman for the force declined further comment on the nature of the ''incident'' and said an investigation was proceeding. It was not yet clear, he said, whether there had been deaths or injuries.
Two military officials said that up to 28 troops had been killed and 11 wounded in the attack on the Salala checkpoint, about 2.5 km from the Afghan border.
The attack took place around in the Baizai area of Mohmand, where Pakistani troops are fighting Taleban militants.
A senior Pakistani military officer said efforts were under way to bring the bodies of the slain soldiers to Ghalanai, the headquarters of Mohmand tribal region.
''The latest attack by NATO forces on our post will have serious repercussions as they without any reasons attacked on our post and killed soldiers asleep,'' he said, requesting anonymity because he was not authorised to talk to the media.
About 40 Pakistani army troops were stationed at the outpost, military sources said. Two officers were reported among the dead.
Nato supply trucks and fuel tankers bound for Afghanistan were stopped at Jamrud town in the Khyber tribal region near the city of Peshawar hours after the raid, officials said.
''We have halted the supplies and some 40 tankers and trucks have been returned from the check post in Jamrud,'' Mutahir Zeb, a senior government official, told Reuters.
Another official said the supplies had been stopped for security reasons.
''There is possibility of attacks on Nato supplies passing through the volatile Khyber tribal region, therefore we sent them back towards Peshawar to remain safe,'' he said.
The border crossing at Chaman in Baluchistan was also closed, Frontier Corps officials said.
Pakistan is a vital land route for 49 percent of NATO's supplies to its troops in Afghanistan, a NATO spokesman said.
Reflecting the confusion of war in an ill-defined border area, an Afghan border police official, Edrees Momand, said joint Afghan-Nato troops near the outpost Saturday morning had detained several militants.
''I am not aware of the casualties on the other side of the border but those we have detained aren't Afghan Taliban,'' he said, implying they were Pakistani Taliban operating in Afghanistan.
The Afghanistan-Pakistan border is often poorly marked, and differs between maps by up to five miles in some places.
The incident occurred a day after US General John Allen met Pakistani Army Chief of Army Staff General Ashfaq Pervez Kayani to discuss border control and enhanced cooperation.
A similar incident on September 30, 2009, which killed two Pakistani troops, led to the closure of one of NATO's supply routes through Pakistan for 10 days.
Nato apologised for that incident, which it said happened when Nato gunships mistook warning shots by the Pakistani forces for a militant attack.
The attack is expected to further worsen US-Pakistan relations, already at one of their lowest points in history, following a tumultuous year that saw the bin Laden raid, the jailing of a CIA contractor, and US accusations that Pakistan backed a militant attack on the US Embassy in Kabul.
The United States has long suspected Pakistan of continuiing to secretly support Taleban militant groups in a bid to secure influence in Afghanistan after most NATO troops leave in 2014. Saturday's incident will give Pakistan the argument that Nato is now attacking it directly.
''I think we should go to the United Nations Security Council against this,'' said retired Brigadier Mahmood Shah, former chief of security in the tribal areas. ''So far, Pakistan is being blamed for all that is happening in Afghanistan, and Pakistan's point of view has not been shown in the international media.''
He called the attack unprovoked and said Pakistan should respond by shooting down Nato aircraft and keeping the supply lines closed.
''Those who say that Pakistan cannot afford a war with the US and Nato, I think we should realise that US and Nato also cannot afford a war with Pakistan.''
Other analysts, including Rustam Shah Mohmand, a former ambassador to Afghanistan, said Pakistan would protest and close the supply lines for some time, but that ultimately ''things will get back to normal.''