MINSK, U.S.S.R. (AP) _ The Soviet Union's three Slavic republics - Russia, Byelorussia and Ukraine - on Sunday declared Mikhail S. Gorbachev's government dead and forged a new alliance.

The agreement dramatically alters the country's political landscape but raises many questions about its legal force, what happens to the remaining Soviet republics, who controls the superpower's nuclear weapons and what role - if any - Gorbachev will play.

The agreement creates the ''commonwealth of independent states'' with its capital in Minsk instead of Moscow and pledges joint administration of foreign relations and military policy including nuclear weapons. It also promises coordination of customs and immigration policies, the news agency Tass said.

The accord defies Gorbachev's desperate appeals to preserve the Soviet Union under a new Union Treaty and says the 74-year-old Soviet Union ''as a subject of international and geopolitical reality no longer exists,'' Tass reported.

Russia's president, Boris Yeltsin, already had stripped the central government of nearly all its functions and resources after the failure of a hard-line coup against Gorbachev in August.

Sunday's agreement, announced after two days of talks by Slavic leaders in the village of Viskuli outside Brest, appears to take Gorbachev even further out of the country's political picture. It represented an attempt by the Slavic leaders to seize the intiative in reshaping the union, which virtually has collapsed since the coup attempt.

''Talks on the preparation of a new Union Treaty have reached a dead end and the process of the secession of republics from the U.S.S.R. and forming the independent states has come to reality,'' Tass quoted the agreement as saying.

The pact, which topped Soviet and Russian television newscasts late Sunday, also sought to assure the West that the new commonwealth would move toward full nuclear disarmament without Gorbachev.

Gorbachev's office said he had no immediate comment, but in a television interview broadcast earlier Sunday he warned of economic and social collapse if republics seceded.

Tass said the republics claimed authority to make the declaration as charter signatories of the 1922 Union Treaty that unified the country. It was unclear, however, whether the act had to be approved by other republics, voters or the already weakened central government.

Tass said the agreement stipulated that other republics could join the commonwealth, but there were was no immediate announcement from them.

Russia, Byelorussia and Ukraine comprise nearly three-fourths of the Soviet Union's 290 million people and possess most of its economic strength, in addition to sharing an ethnic heritage as the country's traditional Slavic heart.

They also have many of the Soviet Union's nuclear weapons, which have been of great concern to the West.

U.S. officials said Sunday the Bush administration, concerned about the political turmoil, would like the Soviet Union's nuclear weapons gathered for safekeeping in the Russian republic. The officials, who spoke on condition of anonymity in Washington, said U.S. Secretary of State James A. Baker III probably would request compliance with the U.S. view when he holds talks next week in Moscow, Ukraine and Byelorussia.

The commonwealth - known as ''sodruzhestvo'' in Russian - will be united in a common economic and military ''space'' that will ''strive for the liquidation of nuclear arms and full disarmament under international control,'' Tass reported.

The Slavic republics said the commonwealth's currency will be the ruble, now virtually worthless. But individual states will be able to issue their own money.

Yeltsin, Byelorussia's Stanislav Shushkevich and newly elected Ukrainian President Leonid Kravchuk signed the commonwealth agreement.

Ukraine, following its vote for independence last week, had rejected Gorbachev's proposed Union Treaty to hold the country together, and the new commonwealth plan keeps the Slavic republics united in a limited manner.

In Washington, White House spokesman Bill Harlow said Yeltsin telephoned President Bush on Sunday to inform him of the action. Asked about the impact of the action on East-West relations, the spokesman said, ''It would be premature to say until we know more about the accords.''

Yeltsin, Kravchuk and Shushkevich were to meet Monday in Moscow with Gorbachev and Nursultan Nazarbayev, president of the powerful Soviet Central Asian republic of Kazakhstan.

Hours before the republics' announcement, Baker said he believed ''there will continue to be efforts to maintain some sort of a center'' in what has been the Soviet Union. Baker spoke on CBS-TV's ''Face the Nation.''

Gorbachev gave his warning of instability in the TV interview, recorded Saturday before the documents were signed.

''I started all this and I have responsibility for the consequences,'' an emotional Gorbachev said, referring to his 6 1/2 -year-old perestroika reform campaign. ''We have reached a kind of limit and after this limit comes instability.''

Gorbachev insisted it would take more than declarations to dismantle the country.

''It's not a structure, not writing or speech. It's reality. It's people. It's life. It's society. This is what the union is,'' Gorbachev said angrily.

The announcement of a radical restructuring of forces came as prominent politicians, disgruntled senior military officers, and supporters of hard-line nationalist groups warned that a new grab for power or even a popular revolt might occur as the economy collapses.