BUDAPEST -- Ukraine formally renounced nuclear arms and ended a long dispute over the former Soviet atomic arsenal when it joined the Nuclear Nonproliferation Treaty on Monday. Kiev's accession to the treaty at a ceremony on the fringes of a European security summit also allowed the START I treaty, under which the United States and the former Soviet Union agreed major nuclear arms cuts, to come into force after years of delay caused by the dispute.


U.S. President Bill Clinton said the ceremony in Budapest "confounded skeptics who had believed the nuclear threat would increase with the breakup of the Soviet Union."


"Together we have helped to beat back the threat of nuclear war and lighten the way to a more peaceful day when the shadow of this destruction has finally vanished from the earth," he said at the ceremony during the summit of the 53-nation Conference on Security and Cooperation on Europe.


The ceremony was also attended by President Boris Yeltsin, British Prime Minister John Major and the leaders of two other former Soviet republics which inherited and then surrendered nuclear arms, Belarus and Kazakhstan.


Ukraine's President Leonid Kuchma, in exchange for signing the accession to the 1970 treaty, received security assurances from the United States, Russia and Britain.


Yeltsin welcomed the step. "We are going along the path of disarmament. Russia believes it should be irreversible," he said. In the three-part ceremony, Yeltsin, Clinton and Major signed a security memorandum with the presidents of Ukraine, Belarus and Kazakhstan; Kuchma handed over a document affirming Ukraine's accession to the treaty, and all of them signed a protocol ratifying START I.


Clinton and Major also called for an indefinite extension of the treaty which formally expires in 1995. "All this is one of the most significant steps towards the complete elimination of nuclear weapons all over the world," Kuchma said. "It initiates a new stage not only in the process of nuclear disarmament but in the realm of international relations generally."


The key to the Ukraine agreement is the security assurances, which have been a contentious issue between Kiev and the nuclear powers.


Kuchma has stressed that Kiev could join the treaty only if the guarantees met the requirements laid down by the Ukrainian parliament, but the final result appeared to fall some way short of Kiev's maximum demands.


The assurances from the United States, Russia and Britain -- three of the world's five declared nuclear states -- are largely those that any state gets when it joins the treaty.


The three powers promised to respect the existing borders of Ukraine. This is important to Kiev because of claims by the Russian parliament for the return of Crimea, which the Soviet leadership transferred from Russia to Ukraine in 1954 and is still largely populated by ethnic Russians.


They also agreed not to use economic coercion on Ukraine, an assurance apparently aimed at soothing fears that Russia might use its economic power against Kiev. The three powers also guaranteed that they would not attack Ukraine except in self-defence or in accordance with the United Nations charter.


Diplomats have ruled out giving Ukraine the kind of blanket security guarantees which NATO states enjoy as the West has refused to offer such assurances to any country outside the alliance, notably former East Bloc nations such as Poland, the Czech Republic and Hungary.


Special treatment for Kiev would have opened the powers to accusations of yielding to nuclear blackmail, diplomats say.


Ukraine has long accepted in principle it should become a non-nuclear state but has held out since 1992 for security and economic guarantees, delaying implementation of START I.


Ukraine inherited well over 1,000 nuclear warheads when the Soviet Union collapsed in 1991. Kiev has now committed itself to being a non-nuclear state under the terms of the 1970 treaty which is aimed at preventing the spread of nuclear arms.