Pakistan suspended the vehicles from the passageway for a security review last week after militants hijacked several trucks whose loads included Humvees bound for the U.S.-led coalition.

On Monday, a dozen or so paramilitary pickup trucks joined a convoy of around 30 vehicles as part of new security measures. The escort trucks carried heavy machine guns and rocket-propelled grenades. Earlier, the transport trucks traveled with little or no security.

Al-Qaida and Taliban fighters, as well as ordinary criminals, are behind escalating violence along the porous Afghan-Pakistan border. The danger has made the Khyber Pass an increasingly perilous 30-mile (50-kilometer) stretch, but one that the U.S. and NATO still rely upon heavily.

It was not possible to confirm exactly what was being transported in Monday's convoy. Military supplies usually travel in sealed, unmarked containers. The route is also a critical commercial trade passage between the two countries.

A U.S. military spokesman in Afghanistan insisted Monday that the suspension had not impacted operations there. "We continue to move supplies through Pakistan to Afghanistan," said Lt. Cmdr. Walter Matthews. "I can't give you the route."

Bakhtiar Khan, a No. 2 government representative in the area, said troops were authorized to shoot "at sight" any militants or other armed attackers who attempt to assault the convoy.

Akhtar Gul was among the drivers who had been waiting for several days to enter the pass. He was pleased to see the armed escorts.

"Previously we had many apprehensions about the security of our lives and the trucks," said Gul, who said he did not know what was in the sealed container he was transporting. "But we hope that now it will be safe."

U.S. and NATO officials in Afghanistan have sought to play down threats posed to the convoys coming through Pakistan, but NATO has said it is close to striking pacts with Central Asian countries that would let it transport "non-lethal" supplies from north of Afghanistan.

In April, NATO concluded a transit agreement with Russia, but it will be of practical use only once the Central Asian nations between Russia and Afghanistan come on board.

Most of the supplies headed to foreign troops in Afghanistan arrive in the southern Pakistani port city of Karachi and are loaded onto trucks for the journey either to the border town of Chaman or through the Khyber Pass.