IN a deadpan declaration, apparently devoid of the emotions of the historic moment, the three Slavic republics of Russia, Byelorussia, and the Ukraine wiped away the Soviet Union on Sunday."We ... state that the USSR is ceasing its existence as a subject of international law and a geopolitical reality," the leaders of the three states declared. But the disappearance of the Soviet Union does not mean the end to its most powerful attribute as a superpower - the tens of thousands of nuclear warheads mounted on missiles, on artillery shells, and on aircraft across the vast territory of the former Union. Those nuclear weapons are now deployed on the territory of the three Slavic republics and the republic of Kazakhstan. For both the leaders of the former Soviet Union and the world, the breakup of the country stirs terrifying visions of the emergence of multiple nuclear-armed states, and even potential use of those weapons in conflicts among them. In Washington, US officials said that one of their main concerns regarding the new Slavic commonwealth was stable control of the former USSR's nuclear arsenal. "We really do run the risk of seeing a situation created there not unlike what we've seen in Yugoslavia, with nukes - with nuclear weapons thrown in," US Secretary of State James Baker III said in a Sunday television interview. "That could be an extraordinarily dangerous situation for Europe and for the rest of the world and indeed for the United States," he added. Thus Mr. Baker, when he travels to the former Soviet Union next Sunday, will urge that all Soviet nuclear arms be withdrawn to Russian soil for safekeeping. Many Soviet nuclear arms are to be dismantled under arms control agreements already signed with the US, and under the unilateral proposals exchanged by President Bush and Mikhail Gorbachev earlier this year. Baker is expected to urge breakaway Soviet republics to pledge adherence to all arms pacts reached by the former central government. Both the Ukraine and Kazakhstan have stated their intention to become non-nuclear states. But both have also balked at the suggestion from Russia that nuclear weapons be sent back to their territory, arguing that the weapons should remain and be destroyed only on their own territory. The joint declaration pledges that the members of the new "Commonwealth of Independent States" will "respect each other's desire to attain the status of a nuclear-free zone and a neutral state. They have decided to preserve the joint command over the common military-strategic space and the single nuclear arms controlling body." The pledge is aimed at reassuring the West of continued control of the nuclear weapons. But Soviet military specialists question how a joint command can exist without central political institution. "Without a central government it is impossible to have centralized control over nuclear weapons," says Sergei Blagovolin, president of the Moscow-based Institute of National Security and Strategic Studies. "It's a fairy tale and nothing more." The bulk of Soviet nuclear weapons are currently on Russian soil but a considerable number are placed in the other three former republics. Official data revealed on Nov. 2 in the Nezavisimaya Gazeta (Independent Newspaper) shows that intercontinental ballistic missiles (ICBMs) are widely dispersed: 4,278 warheads are located in Russia, 1,240 in the Ukraine, 1,040 in Kazakhstan and 54 (all mobile SS-25s) in Byelorussia. Air-launched cruise missiles are also widely distributed, with 367 warheads in Russia, 168 in Ukraine and 320 in Kazakhstan. In addition to these strategic weapons, thousands of tactical nuclear warheads mounted on artillery shells, in short-range missiles, and on bombs to be dropped by aircraft are in the Ukraine. Mr. Gorbachev has pledged to unilaterally destroy all such weapons. Ukrainian defense officials recently told their Russian counterparts that they will seek joint command over tactical as well as strategic weapons. The former Soviet government, including the military, may no longer be able to control the process of dispersion of nuclear weapons, some in Moscow fear. Gorbachev government officials are looking to the West to link recognition of the independence of the Ukraine and Byelorussia, as well as prospects for economic aid and participation in international organizations, to their fulfillment of the strategic arms treaty and the pledge to destroy tactical weapons, preferably under international supervision. At talks held in Washington recently between Soviet and US officials, the main topic was "to maintain strategic stability in a situation of disintegration of the union and fragmentation of our military structures," says Blagovolin, a participant in those discussions. "Only a very strong and very definite pressure from the US, from NATO countries, from the European Community will help us escape from this situation," he said.