The United States has launched exploration with the Soviet Union of President Leonid I. Brezhnev's Oct. 6 proposal for curbs on medium-range nuclear weapons in Europe, a senior State Department official said last night.

The official, who declined to permit use of his name in a meeting with reporters, said the United States is seeking to discover whether the Soviets will agree to curb the numbers or development of their new SS20 missile, which is replacing less powerful and less accurate models.

Secretary of State Cyrus R. Vance has put questions on the matter to Moscow through Soviet Ambassador Anatoliy F. Dobrynin, according to the official, but the answers have not yet been received.

If the Soviets are unwilling to reduce their SS20 deployments, as Carter administration officials suggest may be the case, then the missile-control plans outlined by Brezhnev would seem to call for an unequal degree of sacrifice by East and West, the official said.

The U.S. position is that modernization of North Atlantic Treaty Organization nuclear missile forces is required to match the Soviet buildup that has taken place. For the Soviets merely to stop their buildup in return for a Western decision not to begin would appear to be a trade "of a rabbit for a horse," reporters were told.

The assumption in high State Department as well as White House circles is that the Brezhnev plan is designed to stop NATO in its tracks, with little sacrifice on the Soviet side.

Nevertheless, the active exploration of the Soviet offer through diplomatic channels is the most persuasive suggestion to date that the Brezhnew plan is not being dismissed out of hand.

A formal reply to Brezhnew's missile offer will be given to the Soviets after more information is gathered and after coordination with NATO allies, according to the official.

While the United States is still investigating the missile offer, Vance and other officials have welcomed Brezhnev's proposal, unveiled at the same time, to reduce the number of Soviet troops and tanks in central Europe.

State Department explorations with the Soviets have established that the announced cutback of up to 20,000 Soviet troops and 1,000 tanks within the next year is an unconditional offer, official said.

On a related matter, the official said that the Soviet Union has been informed of the U.S. plan to recommend to Congress next week that most-favored-nation trade benefits be granted to the People's Republic of China, Moscow's archrival. High State Department officials, at least, have not ruled out granting similar benefits later to the Soviet Union, although no decision, has been made to do so.

The senior State Department official also said, in the most explicit comment so far on U.S. District Court Judge Oliver H. Gasch's ruling against U.S. termination of the Taiwan defense treaty by presidential order, that "very serious problems" would result if the decision were allowed to stand. However, he predicted that it would be overruled in a higher court.

The official also sought to lay to rest reports that an unannounced set of anti-Soviet measures, including closer U.S. ties to China, had been adopted by President Carter three weeks ago at the time of his decision regarding Soviet troops in Cuba.

Many options were considered for U.S. counteraction to the Soviet brigade, he said, but no major actions were approved except for those announced at the time.