The United States is negotiating long-term use of a major military base in Uzbekistan to expand the global reach of American forces, despite a brutal government crackdown on protests there last month, Bush administration officials said.

The talks have gone on behind the scenes for several months but have become more awkward for the administration since last month's unrest, which produced the heaviest bloodshed since the Central Asian country left the Soviet Union in 1991. Human rights advocates argue that a new pact would undermine the administration's goal of spreading democracy in the Islamic world.

The U.S. military has relied heavily on Uzbekistan since 2001 in operations in Afghanistan, but on a temporary basis. U.S. Special Operations Forces, intelligence and reconnaissance missions, and air logistics flights all use the Karshi-Khanabad (K2) airfield in southeastern Uzbekistan, according to an official report on U.S. basing.

Now, as the Pentagon carries out a repositioning of U.S. forces overseas, the Bush administration finds itself pursuing the strategic and geopolitical benefits of the Uzbekistan base even as it expresses deep concern about the country's political repression and worries about the risk of American troops caught in widening civil unrest.

"Access to this airfield is undeniably critical in supporting our combat operations" as well as humanitarian deliveries, said Pentagon spokesman Bryan Whitman, who said the United States has paid $15 million to Uzbek authorities for use of the airfield since 2001.

"When you look at the totality of what Uzbekistan has been doing, they've been a very valuable partner and ally in the global war on terror," he said. Asked about the talks on long-range use of the base in Uzbekistan, Whitman said he "wouldn't want to characterize any of our discussions with other governments." But he added: "Clearly, our continued engagement we feel is pretty important."

Yet senior State Department and Pentagon officials said last month's killings of protesters by security forces has led to a high-level review of the military relationship and raised questions about whether, in the long run, "Uzbekistan is the right place for us to be," a senior State Department official said. "No one wants our troops in the middle of someone else's civil conflict or issues," the official said, speaking on the condition of anonymity because of the negotiations.

U.S. officials are concerned that U.S.-trained military units might have participated in the Uzbekistan government's suppression of unrest in Andijan on May 13. U.S. senators including Sen. Patrick J. Leahy (D-Vt.) and human rights advocates say they are pressing the administration to investigate that possibility -- and to stop any talks on military basing until Uzbekistan agrees to an international probe of the killings. Uzbek security forces opened fire on crowds in Andijan that included anti-government demonstrators, Islamic militants and prisoners freed in a jail break.

Pentagon and State Department officials said yesterday that they do not know which Uzbek units were involved in the incidents. The U.S. military has trained some Uzbek special forces and border guard units.

An investigation would most likely show that Uzbekistan authorities "used a level of force that was completely unjustified and they killed many innocent civilians," said Sen. John E. Sununu (R-N.H.). Sununu, Sen. John McCain (R-Ariz.) and Sen. Lindsay O. Graham (R-S.C.) visited Uzbekistan this week but were denied meetings with the government. Based on reports of U.S. Embassy officials there who gathered eyewitness accounts, Sununu believes between 500 to 1,000 people were killed in the unrest and that Uzbekistan Special Forces and regular security forces were involved.

The senators said U.S. military and other relations with Uzbekistan -- including the use of the K2 base -- must be reevaluated in light of Andijan, which Graham called a "massacre."

"Efforts to bring about democracy have hit a wall and are going backwards," he said. "We have a military interest in maintaining our base in that country," but also in "restricting our relations with brutal governments," said McCain, saying the Uzbeks "must understand" that the Andijan events "come with real consequences."

"I would not be comfortable making a long-term commitment" on use of the air base, said Sununu, urging the Pentagon to consider other options -- such as bases in Afghanistan and Kyrgyzstan, in case the United States decides to or is forced to leave.

Officials said it is highly likely that the United States will continue to suspend funds for military purchases and training for Uzbekistan this year, as it did last year, because the State Department could not certify the country was making substantial progress in human rights.

"Before Andijan it was complicated. After Andijan it's become very, very touchy," said a second senior State Department official, who spoke only anonymously because of the sensitivity of the issue. Still, compensation for the airfield would continue, and possibly grow substantially if agreement is reached on long-term use, which could involve building up the base's infrastructure.

For now, the talks between administration and Uzbek officials have not intensified to the level of formal diplomatic negotiations. Officials who describe them said the talks may slow because the Uzbekistan government has limited ties following the unrest. "Uzbekistan is retreating into a hard shell," said another senior State Department official. "Talks will go on for some time." In recent weeks, Uzbekistan has restricted U.S. night and cargo flights in and out of the base, U.S. officials said.

Since the 2001 terrorist attacks, the administration has expanded military aid, cooperation and arms sales to other nations, including some that have been cited by the State Department for poor human rights records.

Senior State Department and Pentagon officials defend stepped-up military cooperation with such countries as necessary for combating terrorism and as a form of engagement that gives the United States the leverage it needs to achieve its goal of fostering democratic change.

Kazakhstan, for example, a vast state stretching from China to the Caspian Sea, grants the United States military airfield access and overflight rights, and is being eyed by the Pentagon for joint military training.

Last month, Secretary of State Condoleezza Rice exercised a waiver to allow continued military aid to Kazakhstan on national security grounds despite what the State Department acknowledged were "numerous steps backward" on human rights, State Department spokeswoman Julie M. Reside said.

She said U.S. military aid "enhances democracy" and so Washington will stay "fully engaged" despite what she outlined as Kazakhstan's many recent regressions -- shutting down newspapers and opposition parties and considering laws that would "paralyze" U.S.-funded nongovernmental groups.

Overall, U.S. Foreign Military Financing (FMF), which provides grants for the purchase of U.S. defense equipment, services and training, has grown by a third since 2001 -- from $3.5 billion to $4.6 billion in 2004, according to State Department figures. Similarly, the United States substantially boosted the training of foreign militaries, with International Military Education and Training (IMET) funds increasing from $57 million in 2001 to more than $90 million in 2004.

While officials say the bulk of FMF grants continue to go to Israel and Egypt, many countries that began receiving such aid anew or for the first time starting in 2001, including Uzbekistan and Pakistan, previously were barred from such military aid because of human rights abuses, nuclear testing, or other problems, according to a report critical of the U.S. military transfers released this week by the World Policy Institute, a nonpartisan think tank based in New York that focuses on arms control.

Critics in Congress and arms-control advocates say the military aid and cooperation are bolstering regimes that oppress citizens and undercutting President Bush's January inaugural pledge to "support democratic movements . . . with the ultimate goal of ending tyranny in our world."

The administration pays "little more than lip service when it comes to countries where abuses by the security forces are routine," said Leahy, ranking Democrat on the Senate subcommittee that oversees U.S. military aid. "Our laws that condition assistance to countries like Uzbekistan, Indonesia and Nepal are not always applied as they should be," said Leahy.

But others argue that the United States has a greater chance to influence those countries by building military relationships. In the 1990s, the United States limited military cooperation by imposing "symbolic sanctions with dozens of countries," said Lincoln P. Bloomfield Jr., who oversaw military assistance programs as assistant secretary of state for political-military affairs from 2001 until earlier this year.

"In the 1990s," Bloomfield said, "you were basically building an electric fence around the United States, and that did not work. We need to engage and engage heavily."