On a day when widespread protests in Kyrgyzstan appear to have overthrown the government, one of US officials' greatest concerns is the status of a critical air base there used as an air hub for the Afghanistan war.

Pentagon officials said Wednesday that Kyrgyz demonstrating against a corrupt government had forced the airport in the capital, Bishkek, to close for about 12 hours. The airport shares space with a US air base known as the Transit Center at Manas. Because of current disruptions, some air cargo jets have had to avoid the base.

“Some personnel and their equipment will be delayed in traversing the Transit Center at Manas,” said an official at the Pentagon who agreed to speak on background. “However, as prudent planning dictates, we will make use of options to ensure that support continues to those conducting operations in Afghanistan.”

The official also said the air base was on a high state of alert.

On Wednesday, President Kurmanbek Bakiyev reportedly left the capital in the presidential plane, but it was not clear if he was fleeing the country or just departing the city. His leadership is in question, and that could have a direct effect on US military operations at Manas.

The US inked a deal last summer with Mr. Bakiyev that allowed continued use of the base. Leading up to that agreement, Bakiyev had first demanded that the US abandon the base but then began intense negotiations over the amount of rent the US was willing to pay for using the base.

As crucial as the base is to American logistical operations, especially in Afghanistan, the US maintained that it would walk away from the deal. But by July, the two countries had signed a deal allowing the US to stay.

The lease deal apparently tripled, from $20 million to $60 million a year.

If Bakiyev has, in fact, fled the country, the terms of the lease could be thrown into doubt, not to mention the US military's long-term relationship with Kyrgyzstan. The US maintains that its air base brings at least $64 million to the local economy.

Manas is home to about 1,100 military personnel, including US, Spanish, and French forces, and another 750 contract workers. As the US expands its presence in Afghanistan – amassing a total 100,000 American troops there by fall – the base has become “the premier air mobility hub” for the International Security Assistance Force in Afghanistan.

Using the base as a hub, military aircraft refueled more than 11,000 aircraft over Afghanistan during 2008 and moved about 170,000 passengers in and out of the country – including troops deploying and redeploying and individual service members traveling to and from the United States on leave.