BISHKEK, Kyrgyzstan — Kyrgyzstan ordered U.S. forces today to depart within six months from an air base key to military operations in Afghanistan, complicating plans to send more troops to counteract rising Taliban and al-Qaida violence.

U.S. Defense Secretary Robert Gates said, however, that the base was not a "closed issue." He had said a day earlier that the United States would consider paying more in rent.

A top U.S. military official said, meanwhile, that neighboring Uzbekistan had granted permission for the transit of nonlethal cargo to Afghanistan — a small victory in the hunt for new supply routes.

The U.S. has had a fraught relationship with Uzbekistan, one of the most politically repressive of the former Soviet states. Most of President Islam Karimov's opponents have been sent to jail or into exile. U.S.-based Human Rights Watch has said that Uzbek prison authorities routinely abuse and torture prisoners.

Anger over Western criticism of a crackdown on an uprising in eastern Uzbekistan prompted the government to evict U.S. troops from an air base near the Afghan border in 2005, leaving the Manas air base in Kyrgyzstan as the only U.S. base in Central Asia.

In recent months, militants have stepped up attacks on convoys traveling through the primary route — Pakistan — pushing U.S. officials to secure alternative, northern routes for Afghan cargo through Central Asia.

The Kyrgyz move to close Manas — a transit point for 15,000 troops and 500 tons of cargo each month heading to Afghanistan — caught the United States by surprise and made the search more urgent.

Kyrgyzstan's president complained when announcing the closure this month that the United States was not paying enough rent for the base.

The announcement came shortly after Russia said it would give $2.15 billion in aid and loans to the impoverished Central Asian nation. U.S. officials suspect that Russia, long wary of U.S. presence in ex-Soviet Central Asia, is behind the decision to shut the Americans out.