Soviet leader Yuri Andropov said tonight that he was prepared to reach an agreement with the United States reducing the number of nuclear warheads in the European theater, a step beyond his previous offers of limiting medium-range nuclear missiles and aircraft.

Balancing his proposal with sharp criticism of the Reagan administration, Andropov said that the U.S. foreign and defense policies are "the root of evil perpetrated in the world, the evil which threatens the very existence of mankind." Those who reject his new proposal would assume "grave responsibility," Andropov said.

In his speech honoring visiting East German Communist leader Erich Honecker, Andropov said that the planned deployment of U.S. medium-range missiles in five Western European nations would create a "chain reaction" in the arms race in which Moscow and its Warsaw Pact allies "will be compelled to take measures in reply." Diplomatic sources here interpreted the remark as a broad hint that the Soviets would put new missiles in East Germany if the U.S. deployment goes ahead as planned next winter.

Andropov's proposal to cut the number of launchers and warheads suggested a slight shift in Moscow's position and an apparent effort to break the impasse at the Geneva talks on medium-range missiles in Europe. In previous proposals the Soviets have insisted that U.S. planes based in Britain carrying nuclear weapons must be included in these talks, but Andropov did not mention those specific planes in his remarks tonight.

In Washington, the State Department announced in a statement that "if the Soviets now recognize that warheads on launchers are the proper unit for account for these (medium-range missile) negotiations, we welcome this as a sign of progress on their part. Our negotiators will, of course, explore this with their Soviet counterparts when the negotiations resume May 17."

But the statement added that Andropov "seems to indicate that the Soviet Union will only reduce its (medium-range) warheads on the basis of corresponding reductions in the British and French strategic nuclear systems. If this is what Mr. Andropov means, then his statement actually represents a hardening of the Soviet position, which had offered reduction" of the Soviet nuclear forces in Europe "to the current number of British and French systems."

Andropov offered in December to reduce the number of nuclear missiles and launchers to 162, or the combined total of the British and French nuclear deterrent forces, if the United States would not deploy new Pershing II and cruise missiles in Europe. Speaking about those proposals, Andropov said that the United States and its allies had objected to the reductions in launchers only because that would leave the Russians with more nuclear warheads than the North Atlantic Treaty Organization.

"All right," Andropov added tonight, "we are prepared to reach agreement on the equality of nuclear potentials in Europe both as regards delivery vehicles and warheads with due account, of course, for the corresponding armaments of Britain and France."

Britain and France maintain independent nuclear forces and have insisted that their arms not be considered as part of the U.S.-Soviet missile talks. But observers here noted that Britain and France are expected to move to multiple warhead missiles soon, thus increasing the number of their warheads. This would make warheads as a unit of measurement in the U.S.-Soviet talks more attractive to Moscow, the observers said.

The shift to negotiations based on the number of the warheads had also been advocated in a report issued last month by the U.S. commission chaired by Brent Scowcroft that studied long-range weapons.

Moscow had earlier raised the possibility of including warheads as one of the main indicators in arms talks. Foreign Minister Andrei Gromyko endorsed the idea during his press conference April 2 when he spoke of warheads as "a more precise, mathematically more accurate expression of the yield of nuclear weapons."

Andropov, in an interview with the West German magazine Der Spiegel last month, also talked about Moscow's interest in reducing both the number of missiles and aircraft and the warheads. Tonight's speech, however, for the first time gave this Soviet attitude the form of a formal negotiating proposal.

Andropov said that Moscow was ready to have the same numbers of launchers and missiles as NATO "in each mutually agreed period." Should the number of warheads on British and French missiles be reduced, "the number of warheads on our medium-range missiles would be cut by as much," he said. The same formula would be applied on medium-range aircraft carrying nuclear weapons in the European theater, he added.

"Thereby," Andropov continued, "an appropriate equality would be maintained between the Soviet Union and NATO both as regards medium-range nuclear weapon delivery vehicles, that is missiles and aircraft, and by the number of warheads carried by them. More than that, this equality would be on a much lower level than now."

"I particularly want to stress that the fulfillment of this new proposal would bring about a situation when in the European part of the Soviet Union we would have much fewer medium-range missiles and warheads on them than before 1976, when we did not have SS20 missiles," Andropov said.

Andropov's proposal was somewhat ambiguous but suggested that he was prepared to reduce the number of SS20 missiles deployed in the European theater to less than 100.

According to Soviet figures, the 162 combined British and French nuclear delivery systems have about 290 nuclear warheads. Excluding Moscow's medium-range nuclear aircraft, the Russians would need about 97 triple-warhead SS20 missiles deployed to have the same number of warheads.

The formula, however, suggested that the Soviets were prepared for an even deeper cut in their SS20 missile force since their aircraft would have to be included in any arms limitation accord.

It was unclear how this formula would work, however, since the U.S. planes were not mentioned in his speech.

While the Soviets have never said how many SS20s they have targeted on Western Europe, western estimates put their number at between 240 and 250. It is estimated that those missiles combined with older Soviet missiles have a total of about 1,100 warheads on missiles in the European theater.