Kicking-off the first phase of the American pull-out from Afghanistan, US has started using Pakistani land routes to withdraw its military hardware from Afghanistan, with 50 shipping containers moved over the weekend.

Two convoys of 25 containers each crossed the Torkham and Chaman border check posts in Pakistan. The containers will be transported to the port city of Karachi to be shipped back to the US.

Two US military convoys, hauling 25 shipping containers each, entered Pakistan via the Chaman and Torkham border posts yesterday, the International Security Assistance Force in Afghanistan said.

The convoys were "part of the US redeployment of equipment from Afghanistan" and traffic is currently "moving both ways through the Pakistan border, with material coming in as well as going out", ISAF said.

Fawad Khan, an official of the shipping company Bilal Associates, told reporters that his firm had received clearance to move the US equipment across the border on Friday.

He said the first convoy of 25 containers cleared by Pakistani customs was on its way to Karachi.

Another Pakistani official, who did not want to be named, said another convoy of containers was expected this week.

"It is a huge operation...Many more containers will follow," he said.

Pakistan will be a key route for the US to withdraw equipment from landlocked Afghanistan as it pulls out most of its troops by the end of 2014.

Pakistan closed the route for nearly seven months after a cross-border NATO air strike killed 24 of its soldiers in November 2011.

The route was reopened after the US apologised for the deaths.

Islamabad has linked the transit of US military cargo with certification that Washington will not transport any hazardous waste or radioactive material.