Although Pakistan has not explicitly threatened to sever the supply lines, Pentagon officials said they were concerned the routes could be endangered by the deterioration of US-Pakistan relations, partly fed by ill will from the cross-border raid that killed Osama bin Laden.

Cables made public by WikiLeaks show that in February 2009 the Obama administration quietly tried to persuade China to open a supply route to US forces in Afghanistan but, as military relations with Washington soured, Beijing said no.

Memories are fresh of Pakistan's temporary closure of a major crossing into Afghanistan in September, resulting in a logjam of hundreds of supply trucks and fuel tankers, dozens of which were destroyed by insurgents.

While reducing the shipment of cargo through Pakistan would address a strategic weakness, shifting supply lines would substantially increase the cost of the war and make the US more dependent on authoritarian countries in Central Asia.

''It's either Central Asia or Pakistan - those are the two choices. We'd like to have both,'' a senior US defence official said, speaking on the condition of anonymity.

US military officials said they had emergency plans ready in case the Pakistan routes became unavailable but that would mean delivering the bulk of the cargo by air, which costs up to 10 times as much as shipping via Pakistan.

As recently as 2009 the US military moved 90 per cent of its surface cargo through Pakistan.

Today, almost 40 per cent of surface cargo arrives in Afghanistan from the north, along a patchwork of Central Asian rail and road routes that the Pentagon calls the Northern Distribution Network. Military planners said they were pushing to raise the northern network's share to as much as 75 per cent by the end of this year.