The Bush administration is preparing to issue a last-ditch warning to Uzbekistan through senior U.S. officials, possibly even President Bush, that Tashkent must allow an international inquiry into the bloodiest unrest since that former Soviet republic gained independence -- or face the danger of unraveling politically like Ukraine, Georgia and Kyrgyzstan, according to U.S. and European officials.

The gambit, the details of which are still being worked out, is part of a bigger-stakes rivalry, reminiscent of the Cold War, that pits the United States and Western allies against Russia and China for influence in Central Asia.

The strategic region has become critical to U.S. efforts to combat terrorism and promote democracy over the past four years. But Washington is on the verge of losing its toehold -- and access to a pivotal Uzbek military base -- because of tensions that have led Tashkent to turn recently to Moscow and Beijing.

The Bush administration is completing plans for an overture to President Islam Karimov, possibly beginning with a Cabinet-level emissary followed by a telephone call from Bush -- if Karimov is open to an international inquiry into the May 13 unrest.

"Certainly, the Uzbekistan government owes its citizens and owes the international community a serious, credible and independent investigation of these events. And we are continuing to push for such an investigation with the government of Uzbekistan and with our partners in the international community," State Department spokesman Tom Casey said yesterday.

A United Nations report issued yesterday concluded that grave human rights violations were committed when Uzbek troops surrounded thousands of demonstrators in downtown Andijan and fired indiscriminately into crowds of men, women and children. Based on credible witness accounts, the crackdown may have amounted to a "mass killing," it said. The report also called for an international investigation, which Karimov has firmly resisted.

The administration hopes to reach out to Karimov by month's end to stress the importance of the U.S.-Uzbek strategic partnership -- which has blossomed since the Sept. 11, 2001, attacks -- while urging the authoritarian government to make a stark political choice so it does not meet the fate of the three other former Soviet republics, U.S. officials say.

"We hope one last push will get Karimov to see that repression leads to instability and the only way out is to embrace freedom. Otherwise, he's on a descending spiral," said a senior U.S. official involved with Central Asian policy who spoke on the condition of anonymity because of the sensitive diplomacy.

For now, Washington has not gone along with a European proposal to issue an ultimatum to Karimov to agree to an international inquiry, with a deadline to reply, or face new sanctions in the form of an arms embargo and a visa ban for diplomats, European envoys said. U.S. officials said they believe that backing Karimov into a corner is not an effective way to win cooperation.

The stakes are high, since the United States has relied on the Uzbek base at Karshi-Khanabad, known as K-2, for military and humanitarian missions in Afghanistan. Uzbekistan, which was one of the first republics to ask Russian troops to leave after the Soviet Union collapsed, has reflected new U.S. influence in Central Asia.

Uzbekistan also symbolizes the central dilemma in U.S. foreign policy over whether democracy or fighting terrorism takes precedence. The Pentagon, facing limited alternatives, wants to keep access to the base; the State Department has advocated a tougher line on political change as the key to prevent further unrest.

The tensions between Washington and Tashkent have offered Russia and China an opportunity to squeeze the United States out of Central Asia. Russia, China and four Central Asian nations -- Uzbekistan, Kazakhstan, Tajikistan and Kyrgyzstan -- demanded this month that the United States declare a date for withdrawing troops and aircraft from bases in Uzbekistan and Kyrgyzstan.

The Shanghai Cooperation Organization, a Eurasian alliance, said a U.S. withdrawal is called for because the active military phase of the Afghan operation is nearing completion.

At the summit, Russian President Vladimir Putin called for increased regional security cooperation. "Russia is trying to take advantage of the situation that the Bush administration's democracy policy has opened for them -- to increase the reliance of Central Asian states on Russia," said Martha Brill Olcott of the Carnegie Endowment for International Peace.

Beijing yesterday called on Washington to honor the request for U.S. troops to withdraw from Central Asia. "It's China going on record and using Russia's shared frustration in Central Asia to say that the U.S. global agenda is one that China is not willing to sign onto," Olcott added.

Karimov has recently visited both countries. "What we're seeing is Karimov shifting his alliances as the political winds change," said Fiona Hill of the Brookings Institution. "He sees willing partners in Russia and China who prefer the status quo."

Lynch reported from the United Nations. Staff writer Ann Scott Tyson contributed to this report.