BUDAPEST -- Upstaging a 52-nation security summit, President Boris Yeltsin on Monday accused the United States and its NATO allies of trying to exclude and isolate Russia.


"Why sow the seeds of mistrust? After all, we are no longer enemies -- we are all partners now," Yeltsin said in an uneasy start to the two-day conference on reducing tensions in an increasingly unstable Europe.


"No major country is going to live by the laws of isolation," he said in an attack on NATO for agreeing last week to draw up conditions for admitting former Warsaw Pact foes. "Any such country will reject having such a game played with it."


But U.S. President Bill Clinton sought to focus attention on fresh moves to end nuclear confrontation on the European continent.


Standing side by side, Clinton and Yeltsin formally put in force the Strategic Arms Reduction Treaty, the first accord ever to reduce long-range nuclear weapons.


"Today we herald the arrival of a new and safer era," Clinton said at a ceremony also attended by leaders of the former Soviet republics of Belarus, Kazakhstan and Ukraine.


Under the START I accord, long-range missiles stockpiled by the United States and the former Soviet Union will be slashed by about one-third, and the stage will be set for U.S. Senate action on the START II accord and its provisions for deeper, 50 percent, reductions.


At the same ceremony, Ukraine, the world's third largest nuclear power, renounced its arsenal of nuclear arms inherited from the Soviet breakup.


Russia and the United States also paved the way for an agreement that would send a multinational peacekeeping force into Nagorno-Karabakh. Thousands have died in the enclave in one of the former Soviet Union's most protracted and insoluble ethnic conflicts.


Yeltsin's remarks at the summit -- which is searching for ways to use the Conference on Security and Cooperation in Europe to defuse European tensions -- underscored the difficulties in agreeing on security measures in the post-Cold War era.


Russia has long sought to make the CSCE the premier security organization in Europe. The United States favors a more assertive CSCE, though not at the expense of the 16-nation North Atlantic Treaty Organization.


Created in 1975, the CSCE was the only institution in which NATO and the Soviet-led Warsaw Pact came together to discuss security and human rights issues.


Speaking ahead of Yeltsin, Clinton sought to head off the Russian's criticisms of NATO membership plans by saying the alliance "will not automatically exclude any nation from joining." But, in an obvious reference to Moscow, he added: "No country outside will be allowed ... to veto expansion.


"As NATO does expand, so will security for all European states," Clinton said. In a slip of the tongue, he also said NATO was "not an aggressive, but an offensive organization." He meant defensive.


But Moscow clearly saw the allies' move to draw in its neighbors as a potentially hostile one.


With Clinton sitting not far away, from him Yeltsin thundered, "Europe has not yet freed itself from the heritage of the Cold War." It was "in danger of plunging into a Cold Peace," he added.


In a slap at Washington, Yeltsin said it was a "dangerous delusion" to think the "destinies of continents and of the world community in general can somehow be managed from one single capital."


Russia's interests, he said, must be considered.


The Western alliance, in ordering a yearlong study of entry requirements, thought it had found a clever formula for meeting East European demands for membership without upsetting Moscow.


The allies promised not to identify front-runners for membership -- Poland and Hungary are considered leading candidates -- nor to set a date for entry. Moreover, it offered Moscow a special dialogue on security and other issues.


Yeltsin said the alliance's search for a new role "should not give rise to new zones of demarcation but rather should consolidate European unity."


If "the intention is to move the frontiers of NATO's responsibility up to Russia's borders, then let me say one thing: It is too soon to bury democracy in Russia," said Yeltsin.


Yeltsin's outburst at the summit marked the third time in less than a week that Russia had used an international forum to spar with the Western powers.


Last Thursday, Foreign Minister Andrei Kozyrev, attending a NATO meeting, unexpectedly attacked the allies' plans to extend membership in Eastern Europe and backed out of a special program of close military and political ties.


On Friday, Russia also vetoed a UN Security Council resolution aimed at stopping Yugoslav fuel supplies reportedly powering a Serb offensive in northern Bosnia.