Defense Secretary Caspar W. Weinberger has asserted that the Soviet Union for years has kept nuclear warheads and missiles on the soil of its Eastern European allies, U.S. officials said in Oslo yesterday.

A senior Defense Department official, who said that he had checked with Weinberger in an effort to clarify confusion surrounding remarks made in Brussels and Bonn this week, confirmed that the secretary was referring to the stationing of Soviet nuclear warheads in Eastern Europe as well as nuclear-capable missile launchers.

The statement appeared to mark the first public information from senior U.S. officials that the Soviets have nuclear warheads in Eastern Europe and not just weapons capable of carrying such warheads.

As if to underscore the degree to which Soviet nuclear capability in Eastern Europe has become another focus of the arms discussion, the official Soviet news agency Tass reported last night that the Czechoslovak Communist Party and government leadership issued an official statement in Prague endorsing "fully and unreservedly" the Soviet statement of May 28 warning that the Kremlin would place nuclear warheads in Eastern Europe if NATO carries out its plans to deploy 572 U.S. cruise and Pershing II missiles in Western Europe.

The Czechoslovak statement said Prague is seriously worried by the planned NATO deployment because it would "create a grave threat to the security of Czechoslovakia" and Europe in general, and it blamed the United States for "showing no desire" to reach an accord in the Geneva talks on reducing strategic arms.

The NATO plan is designed to counter the presence of Soviet medium-range SS20 missiles based in the European part of the Soviet Union and targeted on Western Europe and is coupled with a pledge to pursue arms reduction talks with the Soviets. NATO has said it will not deploy the missiles in Europe if Moscow dismantles its SS20s.

Weinberger's disclosure, hinted at earlier this week, that Soviet nuclear warheads have been in Eastern Europe for some time appeared to be an attempt to play down the new Soviet warning. It took many observers by surprise, since it previously had been assumed that the Soviets would not want to run the risk of maintaining nuclear warheads outside their own territory. But Weinberger dismissed their surprise, saying the presence of nuclear weapons in Eastern Europe has been "common knowledge in the United States."

Defense officials said the nuclear weapons in Eastern European countries, which they did not specify, include modern SS21 and SS23 missiles with ranges of 500 miles or more as well as older Frog and Scud missiles and nuclear artillery shells. They said the newer generation of missiles has been present in Eastern Europe for at least three years and possibly longer.

At a briefing in Brussels Wednesday, a U.S. official discussing the Soviet weapons in Eastern Europe, said, "This is not a new development."

Earlier that day, Reuter reported that Canadian Adm. Robert Falls, the chairman of NATO's military committee, appeared to disagree with that assessment when he said he had no evidence that preparations had been made to deploy the newer Soviet missiles in Europe.

According to studies published in the past 10 years by the authoritative International Institute of Strategic Studies in London, Warsaw Pact countries have had Frog missiles since 1957 and Scuds since 1965, but nuclear warheads for the missiles were believed to be "held in Soviet custody."

Despite Weinberger's assertions, East European sources in Moscow said categorically that the Soviet Union does not have nuclear warheads in Europe. Well-informed sources there said that it would be impossible to keep such deployment secret because of complicated agreements among officers in Warsaw Pact armies and that it would have leaked out if such warheads were deployed outside Soviet borders.

The issue arose last week, after Moscow's threat to counter the planned NATO deployment with its own nuclear missiles in Warsaw Pact countries. Opponents of the NATO missile plan within West Germany also use that argument, warning that the Soviets would respond by upping the nuclear ante in East Germany.

Western leaders initially condemned the Soviet threat and vowed western solidarity in the face of it. As late as Wednesday Adm. Falls was calling the threat serious and destabilizing.

But a few hours later, Weinberger was belittling the threat, saying the Soviets were proposing doing something they had already accomplished, and so their threats were not significant. But when reporters asked whether the mobile missiles in Eastern Europe would be included in the Geneva negotiations on intermediate range missiles, Weinberger seemed unprepared for the question.

"I did not know the precise boundary limits on the kilometer range and there's an intermediate range, there's a longer-range intermediate range, and a whole raft of different subjects," Weinberger said. "But what our desire is to do is to eliminate all the missiles we possibly can in each of these categories."

In press conferences in Bonn and Brussels this week, Weinberger at times seemed to be sidestepping the question of actual nuclear weapons in Eastern Europe, speaking instead of "nuclear-capable" aircraft and missiles. Today, however, a spokesman for Weinberger said there was no intended ambiguity and that the Soviets had had nuclear weapons outside their country for many years. He said he did not know exactly how many, nor did he specify whether the Soviet nuclear warheads actually were on missiles.

Defense officials denied that this week's comments represented a revelation, but they could not cite any previous American discussion of nuclear weapons in the Warsaw Pact countries. They said the weapons' presence was "implied" in a Pentagon publication entitled "Soviet Power."

"There's been no hiding of any of this knowledge that I know of," Weinberger said in his Brussels press conference. "It's common knowledge in the U.S. that these missiles are in Eastern Europe."