The U.S. military has suspended shipments of equipment out of Afghanistan through Pakistan, citing protests that posed a threat to the safety of truck drivers, officials said.

The move, announced on Tuesday, came after club-wielding activists in northwest Pakistan forcibly searched trucks for NATO supplies in protest over U.S. drone strikes in the tribal belt.

In recent days, there have been anti-U.S. demonstrations in Pakistan calling for an end to the American drone program that targets militants and has also killed more than 300 civilians since 2008, according to the Bureau of Investigative Journalism in London.

As a result, U.S. officials said they had ordered truckers under U.S. contract to park temporarily at holding areas inside Afghanistan to avoid going into Pakistan.

Pentagon spokesman Mark Wright said the order affects outgoing shipments that the military calls "retrograde cargo" — equipment and other goods being sent home from military units as the units' numbers are reduced in Afghanistan.

"We have voluntarily halted U.S. shipments of retrograde cargo through the Pakistan Ground Line of Communication (GLOCC) from Torkham Gate through Karachi," Wright said in a statement. He was referring to the main overland route used by the Americans and NATO to withdraw military hardware from Afghanistan, as part of a troop pullout set to wrap up by the end of 2014.

"We anticipate that we will be able to resume our shipments through this route in the near future," he said.