Defense officials say they are examining options for supply routes through a semicircle of nations from Central Asia to the Persian Gulf that could be used in place of the strategic air base in the former Soviet republic of Kyrgyzstan.

Defense officials said planning to substitute for Manas is a preliminary hedge in case the Bishkek government makes good on a threat to expel the United States from a hub serving about 15,000 US personnel coming and going from Afghanistan each month, along with 500 tonnes of goods.

Defense officials spoke on condition of anonymity because the plans are preliminary and the United States is still negotiating with Kyrgyzstan about continued use of the base. Several officials said that dispute is likely to come down to money: Either the United States agrees to a significant increase in rent or Kyrgyzstan will yield to Russian pressure to kick the US out.

Asked about the Kyrgyz situation in an appearance at the State Department, Secretary of State Hillary Rodham Clinton said it was troubling but would not impede US plans to expand its military presence in Afghanistan. She said the Pentagon was "conducting an examination as to how else we would proceed" in the event the Manas air base is no longer available.

"It’s regrettable that this is under consideration by the government of Kyrgyzstan," Clinton told reporters, "and we hope to have further discussions with them. But we will proceed in a very effective manner no matter what the outcome of the Kyrgyzstan government’s deliberations might be."

White House press secretary Robert Gibbs called the Manas base a vital hub for Afghanistan and said the United States is trying to "remedy" the problem with Kyrgyzstan.

The United States set up Manas and a base in neighboring Uzbekistan in 2001 to back operations in Afghanistan. Uzbekistan expelled US troops from the base on its territory in 2005, leaving Manas as the only US military facility in the immediate region.

Besides opening new supply routes from Uzbekistan and other Central Asian nations, another potential option — although one with significant logistical problems — would be a new air supply route from the United Arab Emirates, one official said.

"It’s just at the point of looking at it. There aren’t cost estimates yet," or other crucial data that the new Obama administration would need, and no option has emerged as a preference, one official said.

"We have to consider that now," the official said of the potential loss of the base, "and we are."

The United States had been seeking additional supply routes into Afghanistan for months before the current impasse, driven largely by worry about the safety of overland routes from Pakistan. Uzbekistan and a neighboring Central Asian state, Kazakhstan, have been part of that planning, officials said.

Military leaders have said in public that they were examining and testing new overland options, but details have been slim. The possibility of renewing ruptured ties with Uzbekistan predated the Manas problem but it gained ground as a result, officials said.

Last year the Uzbek government offered to re-establish a US supply route into Afghanistan but no deal went through, according to a former US official who was closely involved in the discussions. The official spoke on condition of anonymity because the talks were never officially concluded.

The US now expects to finish a deal with Uzbekistan by spring to carry what the military calls non-lethal supplies into Afghanistan by commercial rail, the official said. The rail route has already been tested, with the US paying Uzbeks to haul lumber, fuel, cement and other supplies.

The US military also is considering ways it can purchase supplies such as bottled water and building supplies locally rather than hauling them from the United States. Buying more locally would save money and scarce space on space on vehicles or planes, with the added benefit of economic stimulus in Central Asia.

This idea also anticipates greater US supply needs inside Afghanistan as the US military presence there expands by as many as 30,000 troops in coming months.