Nato helicopters attacked a military checkpoint in northwest Pakistan, killing up to 28 troops and prompting Pakistan to shut the vital supply route for Nato troops fighting in Afghanistan, Pakistani officials said.
The commander of NATO-led forces in Afghanistan, General John R. Allen, said he had offered his condolences to the family of any Pakistani soldiers who "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.
The US embassy in Islamabad also offered condolences. "I regret the loss of life of any Pakistani servicemen, and pledge that the United States will work closely with Pakistan to investigate this incident," ambassador Cameron Munter said in a statement.
It is "highly likely" that Nato was behind the raid, a Nato spokesman has told the BBC. Brigadier-General Carsten Jacobson said Nato was investigating how the incident occurred and sent condolences.
The attack comes as relations between the United States and Pakistan, its ally in the war on terror, are already badly strained following the killing of al Qaeda leader Osama bin Laden by US 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.
A Pakistani military spokesman confirmed Saturday's pre-dawn cross-border attack in the tribal region of Mohmand and said casualties had been reported, but gave no details.
"Nato helicopters carried out an unprovoked and indiscriminate firing on a Pakistani check post in Mohmand agency, casualties have been reported and details are awaited," the spokesman told Reuters.
Two military officials said that up to 25 Pakistani troops had been killed and 14 wounded in the attack on the Salala check post, about 2.5 km (1.5 miles) from the Afghan border.
"We have heard about heavy casualties but can't provide you with the exact number of casualties of our troops as the post is far away, located in the mountains and is difficult to reach at the moment," a military spokesman in Peshawar said.
The attack took place around 2am (9pm Irish time) in the Baizai area of Mohmand, where Pakistani troops are fighting Taliban militants.
Another 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.
The commander of Nato-led forces in Afghanistan, General John Allen, said that he offered his condolences to families of any Pakistani soldiers who "may have been killed or injured" during the incident.
Gen Allen said the "incident" had his personal attention and he was committed to a thorough investigation.
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.
Pakistan is a vital land route for 49 per cent of Nato's supplies to its troops in Afghanistan, a Nato spokesman said.
The incident occurred a day after Gen Allen met Pakistani Army Chief of Army Staff General Ashfaq Pervez Kayani to discuss border control and enhanced cooperation.
The Afghanistan-Pakistan border is often poorly marked, and differs between maps by up to five miles in some places.
A similar incident on September 30th, 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.
Reuters