The Soviet Union warned tonight that it might move to introduce nuclear weapons into Eastern Europe if the United States deploys new Pershing II and cruise missiles in five West European countries.

On the eve of the Williamsburg summit of seven major noncommunist industrial countries, the Soviet government said it found "it necessary to warn with utter clarity" that it was determined to take "timely and effective" measures to counter "an additional threat to the security" of the Soviet Bloc posed by the new U.S. deployment.

"The decision of the United States and NATO to start the deployment of new American missiles in Europe," the Soviet government statement said, "if it will be carried out, will force the Soviet Union to reconsider the decision it adopted last year concerning the unilateral moratorium on the further deployment of medium-range systems in the European zone.

"The need would also arise to implement, on arrangement with other Warsaw Treaty member countries, other measures as well as to deploy additional means with the aim of creating the necessary counterbalance to the growing grouping of U.S. forward-based nuclear weapons systems in Europe and the nuclear arms of other NATO countries.

"It would also be necessary, as it has been repeatedly warned by the Soviet side, to take other necessary reply measures with a view to the territory of the United States itself."

The Soviet statement came a day after President Reagan said that deployment of the Pershing II and cruise missiles was necessary to prod Moscow to negotiate seriously for reductions in nuclear weapons.

Tonight's statement is the most authoritative to date as to the types of countermeasures the Russians are contemplating prior to the deployment, which is scheduled to begin in December. Political observers here said it was impossible to assess whether the threatened steps are merely a part of the Soviet propaganda offensive against the U.S. deployment or whether they indeed constitute Moscow's strategic policy.

Calling on the United States and its NATO allies to renounce the plans for the deployment of the new missiles, the Soviet government declared "that it is still not late to stop the dangerous aggravation of the situation" and expressed "hope that the western allies will carefully weigh the consequences that would be an inevitable sequence to the implementation of their plans to deploy new American missiles in Western Europe."

The timing of the statement, published in Saturday's edition of the Communist Party newspaper Pravda but distributed tonight by the Tass news agency, suggested that Moscow wanted to influence the Williamsburg summit, which opens Saturday.

Although there have been speculations and hints about possible introduction of new Soviet weapons into Eastern Europe in response to the U.S. missile deployment, tonight's statement for the first time explicitly raised such a course of action as one of the steps contemplated by the Russians.

The Soviet Union has never deployed its nuclear arms on foreign soil, although it did attempt to do so in Cuba in 1962. Moscow is said to have avoided placements in Eastern Europe because of doubts about the reliability of its allies there.

There was speculation in well-informed circles here following the visit to Moscow by East German leader Erich Honecker that the Soviets would deploy nuclear aircraft and possibly some short-range nuclear rockets in East Germany if the deployment of American medium-range nuclear weapons begins.

Western diplomats here have tended to dismiss such warnings as part of the Kremlin's propaganda drive to put pressure on President Reagan through West European leaders for an arms-control agreement in Geneva that would involve Washington's forgoing the deployment of the Pershings.

Political observers here could not immediately assess the significance of tonight's statement. One view was that Moscow may believe that the threat to resume deployment of SS20s in the European part of Russia and also introduce nuclear weapons onto the territory of some East European states would strengthen the hand of those western politicians who have been urging Reagan to reach an accommodation with Moscow.

By asserting that it was Reagan who was "obstructing" progress at the Geneva talks and that the deployment of American weapons was already producing Soviet countermeasures, the Russians appear to be directing their remarks at West German Chancellor Helmut Kohl and Canadian Prime Minister Pierre Elliott Trudeau. Both are known to favor a more conciliatory line toward Moscow.