The US has stopped shipping military equipment out of Afghanistan, citing the risk to truckers from protests along part of the route in neighbouring Pakistan.

The anti-US demonstrations in Pakistan are calling for an end to drone strikes aimed at militants. US officials have responded by ordering truckers under US contract to park at holding areas inside Afghanistan.

Pentagon spokesman Mark Wright said the order affects shipments of equipment and other goods being sent home from military units as their numbers are reduced in Afghanistan.

"We are aware protests have affected one of the primary commercial transit routes between Pakistan and Afghanistan," he said. "We have voluntarily halted US shipments of retrograde cargo … from Torkham Gate [on the Afghan-Pakistan border] through Karachi to ensure the safety of the drivers contracted to move our equipment."

Many supplies coming into Afghanistan for use by remaining troops were redirected via alternate routes long ago, going through other countries, because of previous problems with Pakistan.

"While we favor shipping cargo via Pakistan because of (lower) cost, we have built flexibility and redundancy into our overall system of air, sea and ground routes to transport cargo into and out of Afghanistan," Wright said.

CIA drone strikes in Pakistan have long been a sensitive subject, with officials regularly criticising them in public as a violation of the country's sovereignty. The issue is more complicated, however, since the government is known to have supported some of the attacks in the past.

Routes through Pakistan have been closed before. The Pakistani government blocked the routes for seven months following US airstrikes that killed two dozen soldiers on the Afghan border in November 2011. Pakistan finally reopened the routes after the US apologised.