In a letter to congressional defense committees, the Pentagon requested "reprogramming" $8.2 billion in funds previously approved to finance more urgent priorities, officials said.

There were "some unanticipated costs that we just didn't foresee," spokesman Captain John Kirby told reporters.

It remained unclear what programs in the defense budget would receive less funding as a result.

A large portion of the request was due to the costs "associated with the extended closure of the ground lines of communication" in Pakistan, Kirby said.

In November, Pakistan shut its border to NATO supply convoys bound for Afghanistan after a US air strike which martyred 24 Pakistani soldiers.

The Pentagon reportedly estimates an alternative network of northern supply routes through Central Asia has generated $2.1 billion in additional fuel costs compared to the shorter Pakistani route.

The request to shift funds was also needed to cover the cost of deploying an additional aircraft carrier to the Gulf region.

"Earlier this year, we did increase the carrier presence there," said Kirby, with the US Navy moving from one to two aircraft carriers present in the area at all times.

The expanded naval deployment came amid rising tensions with Iran, which in December and January threatened to block the Strait of Hormuz in response to increased international pressure over its nuclear program.

The US Air Force also had to pay for more "flying hours" partly related to the closure of the Pakistan routes, as the military has had to rely more on cargo aircraft to ferry in supplies to the US-led force in Afghanistan, officials said. (AFP)