NATO helicopters attacked a military checkpoint in northwestern Pakistan on Saturday, killing as many as 25 troops and prompting Pakistan to shut the vital supply route for NATO troops fighting in Afghanistan, Pakistani officials said.

The attack comes as relations between the United States and Pakistan, its ally in the war on terrorism, are already badly strained following the killing of al-Qaida 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.

A Pakistani military representative confirmed Saturday's predawn 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 representative told Reuters.

Two military officials said that as many as 25 Pakistani troops had been killed and 14 wounded in the attack on the Salala check post, about 2.5 kilometers (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 representative in Peshawar said.

The attack took place around 2 a.m. (2100 GMT) 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 authorized 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.

Pakistan is a vital land route for 49 percent of NATO's supplies to its troops in Afghanistan, a NATO representative said.

A spokesman for the NATO-led International Security Assistance Force in Kabul said the coalition there was aware of an incident and was gathering more information.

The incident occurred a day after U.S. Gen. John Allen met Pakistani Army Chief of Army Staff Gen. Ashfaq Pervez Kayani to discuss border control and enhanced cooperation.

The Afghanistan-Pakistan border is often poorly marked, and differs between maps by as much as five miles in some places.

A similar incident on Sept. 30, 2009, which killed two Pakistani troops, led to the closure of one of NATO's supply routes through Pakistan for 10 days.

NATO apologized 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 U.S.-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 U.S. accusations that Pakistan backed a militant attack on the U.S. Embassy in Kabul.

An increase in U.S. drone strikes on militants in the last few years has also irritated Islamabad, which says the campaign kills more Pakistani civilians in the border area than activists. Washington disputes that, but declines to discuss the drone campaign in detail.

This is an attack on Pakistan's territorial sovereignty, said Masood Kasur, governor of Pakistan's northwestern Khyber-Pakhtunkhwa province.

Such cross-border attacks cannot be tolerated anymore. The government will take up this matter at the highest level, and it will be investigated.

(Additional reporting by Saud Mehsud and Jibran Ahmad; Writing by Augustine Anthony and Chris Allbritton; Editing by Nick Macfie)