The closure would place a strain on US supply lines at a time when President Barack Obama is preparing to nearly double the 36,000-strong force in the country and amid increasing attacks on supply routes through Pakistan.

About 15,000 people and 500 tonnes of cargo move in and out of the Manas air base every month supporting the NATO force in Afghanistan as well as the Afghan army, the Defense Department said on Wednesday.

"It's of concern but it's certainly not a disaster," William Nash, a retired Army general, told AFP.

Nash said there were some "potential sites around the area" and that it was likely that the United States would be able to use bases or roads elsewhere in Central Asia.

The Kyrgyzstan base remains the only US outpost in Central Asia after the US military was evicted from Uzbekistan in 2005 following Washington's condemnation of Tashkent over human rights violations.

But the Pentagon on Wednesday played down the impact of losing use of the Kyrgyz base, saying it would not in anyway undermine the military's operations in Afghanistan.

"The United States would certainly be able to continue operations in Afghanistan if we did not have that facility," Pentagon spokesman Bryan Whitman said.

"We are talking about the United States military, the most flexible, adaptable, capable, innovative military in the world," he said.

US officers in charge of transportation were already exploring other routes to supply the 36,000 troops deployed in Afghanistan after repeated attacks on a key road in Pakistan used to ferry supplies through the Khyber Pass to Afghanistan.

As much as 80 percent of US military supplies to Afghanistan -- from fuel to heavy equipment -- pass through Pakistan, much of it over a single road that threads through the Khyber Pass linking the two countries.

General David Petraeus, head of US Central Command that oversees the region, and General Duncan McNabb, head of the Transportation Command, have in recent months held talks with Central Asian states to explore possible alternative supply routes to take pressure off the Pakistan link.

A spokesman for Central Command said the military had a range of choices to adjust its supply efforts.

"It is certainly a base that we do depend on and that we want to continue to use, but we never put all our eggs in one basket and we have multiple options to continue to supply the troops," Major John Redfield said.

The Manas base, operated by about 1,000 troops including small French and Spanish contingents, was set up to support coalition forces fighting to oust the Taliban regime in Afghanistan in the wake of the September 11, 2001 attacks.

With no base in Central Asia, US hubs in Kuwait and Qatar might have to absorb much of the flight traffic in troops and cargo -- which would also carry a higher price tag, officials said.

But US officials said it was still possible Kyrgyzstan would pull back and agree to allow the Americans to keep using the Manas base.

"If necessary, we will obviously do contingency planning," said State Department spokesman Robert Wood said.

"But, right now, what we are engaged in are discussions with the Kyrgyz authorities to deal with this question of the base."

Although Russia was seen as pushing Kyrgyzstan to close the base in a bid to reassert its influence in the region, Moscow insisted it was ready to assist Washington in securing transit routes.

Russia in recent days had passed a "positive answer" to Washington on its requests for Moscow's help, Russian Deputy Foreign Minister Grigory Karasin said.

Some reports have suggested the assistance could include sending non-lethal supplies across Russian territory by rail.

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