The Central Asian nation of Uzbekistan (search) has ended its agreement allowing U.S. military aircraft and personnel to use an air base that has been an important hub for American military operations in Afghanistan, administration officials said Saturday.

No reason Uzbekistan was evicting U.S. forces from Karshi-Khanabad air base (search), commonly referred to as K2, was offered by either the State Department or the Defense Department. The Washington Post, which first reported the eviction notice, said no reason was given by Uzbekistan and that U.S. forces would have six months to leave.

The New York Times reported Saturday on its Web site that a State Department official cited the abrupt action as a response to a United Nations operation to take hundreds of Uzbek refugees from the region.

More than 400 people who had fled to Kyrgyzstan after an Uzbek uprising in May were flown Friday to a refugee camp in Romania. The Uzbek government had sought their return.

The U.S. Embassy in the Uzbek capital of Tashkent (search) received the diplomatic note terminating the agreement late last week, State Department spokeswoman Nancy Beck said. A Pentagon spokesman, Glenn Flood, said the notice was received Friday.

"This is a bilateral agreement between two sovereign nations, and under that agreement either side has the option to terminate that agreement," Beck said. The State Department had no further comment, she said.

The Uzbek government in recent months had tightened restriction on use of the base, including banning night flights.

"We have to step back and look at our options now and see where we go from here," Flood said. "That airfield has been very important for our operations in Afghanistan" — humanitarian as well as military.

K2 has been a critical staging point for U.S. military operations in Afghanistan since the earliest days of the war, which began in October 2001.

More recently, the base has been used to move supplies, including humanitarian aid, into northern Afghanistan. It also is a refueling point for transport planes.

The eviction notice came just days after Defense Secretary Donald H. Rumsfeld (search) returned from a Central Asia visit to two Uzbek neighboring states, Kyrgyzstan and Tajikistan. Officials in Kyrgyzstan affirmed to Rumsfeld that U.S. forces can continue to use Manas air base for as long as the Afghan war requires.

U.S. forces do not use any bases in Tajikistan, which shares a long border with northern Afghanistan. The Pentagon has an arrangement that permits U.S. planes to refuel there under certain circumstances.

During his trip, Rumsfeld said he did not believe U.S. operations in Afghanistan would be hurt if the Uzbek government denied continued use of K2 because there are other air base options in the region.

"We're always thinking ahead. We'll be fine," Rumsfeld said on Monday.

In early July, a regional organization led by Russia and China issued a statement calling for the U.S. to set a timetable for withdrawing its forces from Uzbekistan and Kyrgyzstan.

Uzbekistan's ties with Washington have deteriorated after the Bush administration joined other Western nations in urging an international investigation into the suppression of a May uprising in the eastern Uzbek city of Andijan.

Uzbek government troops fired on protesters in the city after militants seized a prison and a government building. Authorities denied that troops fired on unarmed civilians and said that 187 people died in the unrest; human rights groups put the figure as high as 750.

Uzbekistan's president, Islam Karimov, who has ruled for 16 years and tolerates no dissent, has blamed the violence on Islamic militants.

He has rejected the demands for an outside inquiry, and, facing Western criticism, has found a strong support in Russia and China. Both of them are wary about the U.S. military presence in the strategic and resource-rich region.