BRUSSELS, Belgium -- President Clinton, hailing "a hopeful and historic breakthrough," announced yesterday an agreement that would finally remove all nuclear weapons from Ukraine -- the world's third largest nuclear arsenal.
The bulk of that arsenal is pointed at the United States from the time when Ukraine was part of the Soviet Union, but it is fear of Moscow that has lately made Ukrainians anxious about giving up the weapons as agreed to under earlier treaties.
The agreement announced yesterday contains guarantees that neither Russia or the United States would launch a nuclear attack against Ukraine. Ukraine will also get hundreds of millions of dollars to help dismantle the nuclear arsenal and considerable assistance in advancing its peaceful nuclear energy program.
Many details about the nuclear removal appeared to remain unsettled yesterday, but a clearly delighted Mr. Clinton said that he would stop off at the Ukrainian capital of Kiev tomorrow to thank Ukraine President Leonid Kravchuk personally before going to Moscow.
The agreement, to be signed Friday in Russia, ended months of difficult three-way negotiations among the United States, Russia and Ukraine. The development is seen by White House officials as a much-needed boost for Russian President Boris N. Yeltsin.
Both Russian nationalists and the Russian military establishment have been strongly opposed to the prospect of any other nuclear powers on Russia's borders. And this agreement, which the Americans pursued vigorously, gives Mr. Yeltsin a tangible proof that his friendship with the Clinton administration actually helps the Russian people.
"It helps show the Russian public that there is some benefit to the relationship," said a State Department spokesman, Michael McCurry.
Likewise, the agreement to dismantle 1,240 nuclear warheads -- two-thirds of them mounted on intercontinental missiles and aimed at the United States -- also underscores what Mr. Clinton has been saying consistently since a speech at Annapolis nine months ago: that what happens in the former Soviet Union directly relates to the safety and well-being of Americans.
Secretary of State Warren M. Christopher, appearing before reporters to discuss the agreement, said it is "clearing up some of the remnants of the Cold War and the nuclear danger that we confronted for such a long time."
President Clinton, who preceded Mr. Christopher at the lectern, also said that dealing with leftover nuclear weapons in the four republics that were once part of the Soviet Union constituted "the most important non-proliferation challenge facing the world."
Kazahkstan and Belarus -- the other two nuclear states besides Russia and Ukraine -- have already made the decision to go non-nuclear. Ukraine's decision, said Mr. Clinton, ensures "that the break-up of the Soviet Union does not result in the birth of new nuclear states which could raise the chances for nuclear accident, nuclear terrorism or nuclear proliferation."
Special concerns existed about Ukraine's keeping nuclear weapons because the country was not well-equipped to handle them. The complicated weapons require constant maintenance to prevent accidents. Fears also existed about potential sales to rogue states.
While the president and his foreign policy advisers were eager to announce the deal, it quickly became clear that much of the details still need to be ironed out.
Mr. Christopher and other administration officials ducked the question of how long the agreement gives Ukraine to rid itself of the weapons. All they would say is that at a "minimum" the weapons would be dismantled in seven years.
The president and his advisers were also a bit vague on the vexing question of whether President Kravchuk actually has the authority to enter into such an agreement with Mr. Clinton and Mr. Yeltsin.
Once before, the administration thought it had an agreement, only to have Ukraine's Parliament balk at ratification.
Asked about this, Mr. Clinton said only: "We have no reason to doubt the ability of the president to keep the commitment that he is prepared to make."
But officials in Kiev were silent on the agreement, crafted to accommodate demands by the Ukrainian Parliament for security guarantees and billions of dollars in compensation in exchange for going non-nuclear.
An official announcement in Kiev said merely that the Moscow talks would focus on "the completion of recent trilateral negotiations." Senior Ukrainian officials called for caution and suggested the final terms of the accord had yet to be determined.
But the prospect of Mr. Kravchuk's traveling to Moscow to sign a nuclear accord with Russia and the United States immediately came under fire from Ukraine's nationalist opposition.
"President Kravchuk has no authority to sign an international document on nuclear weapons," a top opposition leader, Vyacheslav Chornovil, told the Interfax-Ukraine news agency. "Nuclear policy is to be worked out by Parliament."
Under the terms of the deal reached yesterday, teams of experts from Russia and the United States are to remove the warheads from the missiles now in Ukraine. The missiles and the warheads would then both be shipped to Russia, where the missiles themselves would be destroyed. The highly enriched uranium on the warhead would be converted to so-called "light" uranium for fuel rods in civilian power plants.
The agreement calls for $1 billion worth of those rods -- the amount estimated to be present in Ukraine's warheads -- to be sent back to Ukraine for use in that nation's nuclear power plants.
In addition, Ukraine, which is in dire need of financial aid, stands to get $175 million in U.S. assistance under a law that underwrites de-nuclearization efforts in order to ensure that they are done safely.