Pentagon spokesman Bryan Whitman expressed gratitude on Thursday for the Uzbek government's cooperation in fighting terrorism, but noted that the Pentagon has not made Karshi-Khanabad air base the sole focus of its basing plan for Central Asia.
"None of our operations are dependent on any single entity," he said.
At the State Department, spokesman Sean McCormack said the agreement allowing US troops to use the base had been "mutually beneficial to both sides" but that the US had taken "steps to secure other facilities".
Uzbekistan says it will reassess Washington's use of an airbase close to its border with Afghanistan due to the changed situation there and payment problems concerning the base, the foreign ministry said.
"The presence of the armed forces contingent at Khanabad was always ... conditional on a direct connection with the conduct of the military operation in northern Afghanistan," the foreign ministry said in a statement.
"Other prospects for the presence of the US armed forces contingent on the territory of Uzbekistan were not considered by the Uzbek side."
Unpaid US bills
In talks with Washington, Tashkent intends to raise a number of financial issues linked to the base, including compensation for ecological damage, payment for infrastructure improvements and unpaid take-off and landing fees of US planes from the base.
On Tuesday, a regional alliance led by China and Russia and including Uzbekistan called for the US and its coalition allies in Afghanistan to set a date for withdrawing from several states in Central Asia.
"In the opinion of the Uzbek foreign ministry, these fundamental considerations should be the basis for discussion of the question of the future presence of the US armed forces contingent at Khanabad"
Uzbek Foreign Ministry statement
The Uzbek Foreign Ministry said that the air base at Karshi-Khanabad was only intended for combat operations in northern Afghanistan during the US invasion of that country.
"In the opinion of the Uzbek foreign ministry, these fundamental considerations should be the basis for discussion of the question of the future presence of the US armed forces contingent at Khanabad," the ministry said.
US pullout deadline
The statement comes after Uzbek President Islam Karimov this week signed a declaration calling for deadlines on the pull-out of US bases from Uzbekistan and Kyrgyzstan, along with other leaders of the Shanghai Cooperation Organisation, a security bloc that comprises China, Russia, Uzbekistan, Kazakhstan, Kyrgyzstan and Tajikistan.
Karimov first granted Washington the right to use the airbase for military operations during its inavsion of Afghanistan in 2001.According to the US military, Uzbekistan hosts at least 800 US troops, while 1,200 troops from the United States and South Korea are in Kyrgyzstan. Some 200 French air force personnel are based in Tajikistan.
Uzbekistan's ties with the US and other Western nations have sharply deteriorated since its harsh suppression of a May uprising in the eastern city of Andijan.
Uzbek authorities say 176 people died and deny that government troops fired on unarmed civilians, but rights activists say as many as 750 may have been killed.
Karimov put restrictions on the US air base - located in southern Uzbekistan about 180km from the Afghan border after Washington joined calls by other Western nations for an international probe into the Andijan massacre.
However, Russia and China expressed support for Uzbek authorities at the time.
Moscow sees the region as part
of its sphere of influence
Meanwhile, Moscow-based Central Asia expert, Arkady Dubnov, said that the Uzbek decision marked a serious strategic setback for Washington.
"From a purely tactical point of view, it's not a disaster as the situation in Afghanistan is more or less under control and they have other bases," he said. "But from a geopolitical standpoint, it is a clear defeat for the United States.
Uzbekistan is the first country to leave the anti-terrorist coalition in Afghanistan and this will boost the influence of Russia and China (in Central Asia)," he said.
Moscow sees the region historically as part of its sphere of influence and China is seeking a dominant role in the region because of its extensive energy resources.