US Secretary of State Condoleezza Rice and Kyrgyz President Kurmanbek Bakiyev signed a brief statement on Tuesday promising open-ended US use of the Manas air base for Afghan operations.
Bakiyev also agreed to a US inquiry into whether past payments for use of the facility might have fallen into corrupt hands.
Rice is in Central Asia to celebrate democratic progress, such as the election that brought Bakiyev to power this year after the ouster of an increasingly authoritarian predecessor.
Rice also wanted to firm up US rights to the base, for which it pays about $40 million a year.
"The duration of the [US-led military] presence directly depends on the situation in Afghanistan," Bakiyev said after meeting with Rice.
Balance of power
Bakiyev has made similar promises on the base before, but has also wavered on the terms. The base has become particularly important since the government of neighbouring Uzbekistan said the US must leave a base there.
Both facilities, plus one in Afghanistan, are used heavily in the four-year-old war in Afghanistan.
"Kyrgyzstan will maintain balanced and equal relations with all the states with which it has cooperated both multilaterally and unilaterally"
Tensions over military bases rose over the summer when the Shanghai Cooperation Organisation, dominated by Russia and China, urged the US military to withdraw its bases from Uzbekistan and Kyrgyzstan.
The move was seen as a joint effort by Russia and China to drive Washington out of the strategically placed, resource-rich region, and Bakiyev initially seemed to go along.
Kyrgyzstan also hosts a Russian base, and the country is still heavily influenced in language, architecture and culture by its years as a Soviet republic.
"We cannot say that Kyrgyzstan prefers Russia over the United States or vice versa," Bakiyev said. "Kyrgyzstan will maintain balanced and equal relations with all the states with which it has cooperated both multilaterally and unilaterally."
US officials said Tuesday's agreement is the first time Bakiyev has put the base rights in writing, and represented his most explicit repudiation of the Russian and Chinese position.