"This is a new day in the long history of Russian-American relations, a day of progress and a day of hope," Bush said during a news conference with Russian President Vladimir Putin.

The United States has about 7,000 warheads.

Putin said Russia would "try to respond in kind" to the U.S. nuclear cuts but did not announce a specific number.

On the topic of missile defense, Putin said Russia's position "remains unchanged."

U.S. officials had played down the odds of a breakthrough on differences over missile defense and the 1972 Antiballistic Missile Treaty, but they did not rule one out and predicted the two leaders ultimately would reach an accommodation on that front as well.

Bush repeatedly has promised to abandon the ABM Treaty unilaterally if an agreement cannot be struck with Moscow. The treaty prohibits much of the testing and military deployment that Bush said is necessary for the U.S. missile defense program.

Putin has significantly softened his position in recent months and talked of a possible accommodation.

Bush also announced Tuesday that the United States and Russia had agreed to work together to share information and expertise to counter the threat of bioterrorism, to improve the physical protection and accounting of nuclear materials, to cut off "every possible source" of nuclear, chemical and biological weapons, and to fight organized crime and drug trafficking

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