The United States has suspended shipments of equipment out of Afghanistan through Pakistan due to protests against US drone strikes that posed a risk to truck drivers.

Pentagon issued a statement on Tuesday, saying Washington decided to stop cargo shipments voluntarily from Torkham Gate through Karachi that the US and NATO troops used for withdrawing from Afghanistan as part of the 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," said Mark Wright, Pentagon spokesman, in a statement. The trucks have been told to wait for now in holding areas in Afghanistan, Wright said.

"The companies that we contract with were getting nervous, And it's getting a little too dangerous for the truck drivers," the defence official, who spoke on condition of anonymity, told AFP news agency.

The US has alternative routes available to the north through Central Asia, but those options take longer and are more expensive.

Latest blow to withdrawal

The delay in removing the equipment was just the latest hitch in a deeply troubled relationship between Pakistan and the US due to CIA drone missile attacks in the country that have been blamed for civilian deaths and sparked public outrage.

US officials said the strikes by unmanned aircraft have taken out dangerous al-Qaeda fighters.

While Pakistani officials publicly criticised the bombing campaign, the government was believed to have backed at least some of the strikes over the years.

The latest shutdown of the Pakistan-Afghanistan border for coalition trucks have lasted more than seven months after a US helicopter accidentally killed 24 Pakistani troops.

The border reopened to supply vehicles in July 2012 after Washington issued an apology.

The Pentagon said it had to send home 24,000 vehicles and 20,000 shipping containers of equipment as of September after more than 12 years of war.

The whole withdrawal will cost an estimated $7bn, according to Pentagon officials.