US Defense Secretary Leon Panetta flew into Kyrgyzstan on Tuesday amid concern in Washington over the future of an air base that serves as a crucial hub for the war in Afghanistan.

The Pentagon chief planned to discuss the Manas base in an unannounced visit to Bishkek as US officials weigh a possible troop presence in Afghanistan beyond 2014, which would require use of runways at the base after a current rental deal expires.

Panetta said he would speak to Kyrgyz leaders about the "importance" of the air base, a vital transit point used to ferry troops to Afghanistan, refuel warplanes and evacuate wounded soldiers.

"I want to thank them and ensure that relationship can continue into the future as well," Panetta told reporters aboard his plane before landing in the capital.

President Almazbek Atambayev has called for changes in the arrangement at Manas after the current agreement runs out in mid-2014.

The visit was part of an effort by the United States to persuade Atambayev to leave the door open to renewing access to the Manas base after the current deal ends, a US defense official suggested.

"We think that there may be some longer-term wiggle room there, so we're not ruling anything out," the official, who spoke on condition of anonymity, told reporters.

"It's important that we underscore to them and to this new president that it is in fact very important to us," said the official.

The US administration also hopes to make the case that supporting the war effort in Afghanistan serves Kyrgyzstan's interests, by boosting stability in the region, the official added.

After Kyrgyzstan threatened to cancel US access to the base in 2009, Washington secured a new deal by agreeing to triple the rent paid to Bishkek. The United States now pays $60 million a year for use of the air field, up from an earlier annual fee of about $17 million, officials said.

As part of the agreement, the facility was renamed the Manas "transit center" to downplay the military operations.

The US presence has irritated Russia, placing Kyrgyzstan at the center of a power rivalry for regional influence.

Manas, which hosts about 1,500 US troops and contractors and a fleet of KC-135 refueling tanker aircraft, operates round-the-clock, with planes transporting thousands of troops and hundreds of tonnes of cargo every month.

Last year, the base oversaw 4,786 refueling flights, with more than 296 million pounds of fuel delivered, according to the Pentagon.