The Kremlin said in a statement that the 1990 pact was suspended "due to exceptional circumstances in relation to the treaty's content that affect the security of the Russian Federation and require immediate measures."

Russia previously had threatened the move because of its opposition to U.S. plans to deploy a missile defense system in Eastern Europe to ward off a potential threat from Iran. Russian officials regard the project as unnecessary because they believe that Iran is many years from developing long-range missiles. And, more critically, military officials here believe the system can -- and probably will -- be used by the United States to peer deep into Russian territory.

Suspension of the Conventional Forces in Europe Treaty will deepen the country's strained relations with its immediate neighbors in Eastern Europe. Russia can now move more tanks and other heavy weapons to its western borders, and officials in Poland, Estonia and other neighboring countries quickly said they deplored the suspension.

But political and military analysts said major redeployments are unlikely. The suspension, they said, was both a symbolic expression of Russian anger over missile defense and a demonstration that the country has returned as an assertive power that must be reckoned with.

NATO called Russia's decision a "disappointing step in the wrong direction." "NATO considers this treaty to be an important cornerstone of European security," said James Appathurai, a spokesman for the alliance.

The White House expressed its disappointment with the Russians. But Gordon Johndroe, a spokesman for the National Security Council, said in a statement, "we'll continue to have discussions with them in the coming months on the best way to proceed in this area -- that is in the interest of all parties involved and provides for security in Europe."

Relations between the United States and Russia continue to slide despite a recent attempt at mitigating the tension when President Bush invited President Vladimir Putin to the summer home of Bush's parents in Kennebunkport, Maine.

Besides the missile defense system, the two countries disagree on the future status of the Serbian province of Kosovo and how severely the international community should react to Iran's nuclear program.

The Kremlin is also deeply hostile to the prospect of countries such as Georgia and Ukraine joining NATO following the accession of Baltic and Eastern European countries. Officials here describe NATO expansion as an aggressive encirclement of Russia and an attempt to isolate the country in its natural sphere of influence.

There is a widespread view here that the United States, which has consistently criticized the pace of Russia's democratic development under Putin, wants to undermine the country's newfound self-confidence in its status as a booming energy superpower. Putin's decision is likely to be viewed not just as a snub of the West, but as further proof that the Russian president has restored the country's ability to assert its independence.

In Russia, the move drew applause from across the political spectrum.

"Russia can't just twiddle its thumbs when it sees the Americans taking root in the Baltic and Caucasus countries and strengthening their positions in East European countries," Gennady Zyuganov, the leader of the Communist Party, told the Russian news agency Interfax. "When NATO's steam engine is directed toward us, we simply must respond."

Russia has long bridled at the failure of NATO countries, including the United States, to ratify amendments to the treaty made in 1999.

The amendments, however, required Russia to withdraw troops from Moldova and Georgia, and some NATO countries refuse to act until Russia withdraws its troops from those former Soviet republics.

Western countries also argued that Russian force levels in the restive republic of Chechnya have at times not been in compliance with the treaty. The amendments would have allowed Russia to bolster its forces in southern Russia but only in return for withdrawal from Georgia and Moldova.

Russia is drawing down its forces in Georgia but charges that NATO violates the treaty because of deployments in Eastern Europe. NATO officials reject that accusation.

In April, Putin pledged to suspend the treaty, arguing that Russia was threatened by U.S. plans to place a radar system and interceptor missiles in the Czech Republic and Poland. In his annual state of the nation address that month, Putin said NATO was "building up military bases on our borders, and, more than that, they are also planning to station elements of anti-missile defense systems."

Russian officials have noted that in 2001, the Bush administration unilaterally pulled out of the 1972 Anti-Ballistic Missile Treaty because it said the pact no longer served U.S. interests.

U.S. officials thought the threat of a Russian pullout from the conventional forces treaty had eased as the two countries continued to discuss a compromise on missile defense in Europe. Before the recent summit between Putin and Bush in Kennebunkport, the Kremlin informed the United States that despite Putin's April statement, it would admit inspectors under the treaty.

But Saturday, Russia said it would end the inspection of its military installations by NATO countries after a formal notification period of 150 days. The Russian Foreign Ministry said in a statement that the suspension "does not imply we are shutting the door to further dialogue."

"If today's message is ignored, the Intermediate-Range Nuclear Forces Treaty will be next," Gleb Pavlovsky, a Kremlin political consultant, said in an interview with Interfax. "A mad arms race in the Caucasus, Caspian and Black Sea regions is underway, and it is being maintained by European and non-European countries, none of them restricted by the" treaty.