An army soldier stands guard near caskets of soldiers killed in a cross-border attack along Pakistan and Afghan during their funeral prayers in Peshawar November 27, 2011. [Photo/Agencies]

Around 40 troops were stationed at the outposts at the time of the attack, military sources said.

Militants targeting NATO forces have long taken advantage of the fact that the alliance's mandate ends at the border to either attack from within Pakistan or flee to relative safety after an attack.

Three Pakistani soldiers were killed last year by NATO gunships. NATO said then that its forces had mistaken warning shots from Pakistani forces for a militant attack.

In the latest incident, a Western official and a senior Afghan security official said NATO and Afghan forces had come under fire from across the border with Pakistan before NATO aircraft attacked a Pakistani army post, killing the soldiers.

"They came under cross-border fire," the Western official said, without identifying the source of the fire.

The Afghan official said troops had come under fire from inside Pakistan as they were descending from helicopters, which had returned fire.

Both officials asked not to be named because the attack is so sensitive.

Pakistan has said the attack was an unprovoked assault and has said it reserves the right to retaliate.

Strained relations

US and NATO officials are trying to defuse tensions but the soldiers' deaths are testing a bad marriage of convenience between Washington and Islamabad.

Many Pakistanis believe their army is fighting a war against militants that only serves Western interests.

Foreign Minister Hina Rabbani Khar spoke with US Secretary of State Hillary Clinton by telephone early Sunday to convey "the deep sense of rage felt across Pakistan" and warned that the incident could undermine efforts to improve relations, the Foreign Ministry said in a statement.

Pakistan shut down NATO supply routes into Afghanistan in retaliation for the incident, the worst of its kind since Islamabad uneasily allied itself with Washington following the September 11, 2001, attacks on the United States.

Pakistan is the route for nearly half of NATO supplies shipped overland to its troops in Afghanistan. Land shipments account for about two thirds of the alliance's cargo.

A similar incident on Sept 30, 2010, which killed two Pakistani service personnel, led to the closure of one of NATO's supply routes through Pakistan for 10 days.

US ties with Pakistan have suffered several big setbacks starting with the unilateral US special forces raid in May that killed bin Laden in a Pakistani town where he had apparently been living for years.

"From Raymond Davis and his gun slinging in the streets of Lahore to the Osama bin Laden incident, and now to the firing on Pakistani soldiers on the volatile Pakistan-Afghan border, things hardly seem able to get any worse," said the Daily Times.

Islamabad depends on billions in US aid and Washington believes Pakistan can help it bring about peace in Afghanistan.

But it is constantly battling Anti-American sentiment over everything from US drone aircraft strikes to Washington's calls for economic reforms.

"We should end our friendship with America. It's better to have animosity with America than friendship. It's nobody's friend," said labourer Sameer Baluch.

In Karachi, dozens of truck drivers who should have been transporting supplies to Afghanistan were idle.

Taj Malli braves the threat of Taliban attacks to deliver supplies to Afghanistan so that he can support his children. But he thinks it is time to block the route permanently in protest.

"Pakistan is more important than money. The government must stop all supplies to NATO so that they realise the importance of Pakistan," he said.