Foreign Ministry spokesman Abdul Basit said: "We have temporarily stopped supplies of NATO forces from Pakistan to Afghanistan after the soldiers were killed by intruding NATO gunships in Pakistani territory.”

The blockade appeared to be a major escalation in tensions between Pakistan and the United States. A permanent stoppage of supply trucks would place massive strains on the relationship between the two countries and hurt the Afghan war effort. Even a short halt is a reminder of the leverage Pakistan has over the United States at a crucial time in the nine-year-old war.

By late afternoon, a line of more than 150 NATO vehicles was waiting to cross the border into Afghanistan, officials said.

"We will have to see whether we are allies or enemies," Pakistani Interior Minister Rehman Malik said of the border incident, without mentioning the blockade.

Three Pakistani soldiers were also injured in the attack by helicopters of the International Security Assistance Force (ISAF) on a military checkpoint in Kurram Agency, an ethnic Pashtun tribal region on the Afghan border. The early-morning attack took place in Teri Mangal village.

NATO said its aircraft entered Pakistani airspace in self-defense and killed "several armed individuals" after air crews believed they had been fired at from the ground.

After striking what was believed to be an insurgent group, "the aircraft received what the crews assessed as effective small arms fire from individuals just across the border in Pakistan," ISAF said in a statement. "Operating in self-defense, the ISAF aircraft entered into Pakistani airspace killing several armed individuals," it said.

A Pakistan military spokesman in a statement said two helicopters from Afghanistan used cannon fire against the outpost of the Frontier Corps located 200 meters inside Pakistan. Troops present at the post manned by six soldiers "retaliated through rifle fire to indicate that the helicopters were crossing into our territory," the spokesman said.

"Instead of heeding the warning, helicopters went on to fire two missiles, destroying the post. As a result, three FC soldiers have embraced shahadat (martyrdom) and three have been injured."

Over the weekend, NATO helicopters fired on targets in Pakistan at least twice, killing several suspected insurgents they had pursued over the border from Afghanistan. Pakistan's government protested the attacks, which came in a month during which there have been an unprecedented number of US drone missile strikes in the northwest, inflaming already pervasive anti-American sentiment among Pakistanis.

Prime Minister Yousaf Raza Gilani told visiting CIA Director Leon Panetta in Islamabad that Pakistan was "profoundly concerned" about the missile strikes and helicopter incursions. "Pakistan, being a front-line ally in the war against terror, expects its partners to respect its territorial sovereignty," he said, according to a statement from his office.

The statement quoted Panetta as saying that Pakistan's sovereignty will be fully respected and the US will try its utmost to provide timely and credible information to Pakistan against any possible terrorist activity on its side of the border to enable Pakistani forces to take prompt action against the troublemakers.

— With input from agencies