The Soviet news agency Tass today called the Reagan administration's reasons for dismissing a Soviet freeze on medium-range nuclear weapons "a gross lie," but there were indications that the Soviets were not closing the door to improved relations with the United States.

Privately, Soviet officials expressed regret that the proposal by Soviet leader Mikhail Gorbachev had been rejected so hastily.

[The Reagan administration, meanwhile, is making no plans to offer its own initiative in response to Gorbachev's moratorium, administration officials said. At the same time, officials took a positive view of Gorbachev's remarks about a U.S.-Soviet summit meeting and said diplomatic contacts on this subject have intensified recently.]

Gorbachev's initiative came up today during a series of meetings between a congressional delegation headed by House Speaker Thomas P. (Tip) O'Neill (D-Mass.) and Soviet officials, according to members of the delegation.

An unusually high-level group of Soviet officials attended a reception for the congressmen tonight. Among those present was Vadim Zagladin, first deputy chief of the Communist Party's international department, who had not been at a U.S. Embassy function in several years, an embassy spokesman said.

Tass issued a lengthy report tonight on the delegation's visit and quoted O'Neill as saying the purpose of the visit was to promote improved Soviet-U.S. relations.

In the Tass report on the U.S. reaction to Gorbachev's move, military analyst Vladimir Chernyshov said that "the U.S. administration wishes neither this, nor the other -- neither the arms reduction, nor the renunciation of the arms buildup."

Gorbachev announced in an interview yesterday with the Communist Party newspaper Pravda that the Soviet Union would halt deployment of SS20 missiles until November and challenged the United States to halt further deployments of Pershing II and cruise missiles in Europe.

The White House rejected the Soviet proposal, saying that a freeze at current levels would leave the Soviets with a clear advantage in missile strength.

The Soviets now have an estimated 414 triple-warhead SS20s, of which about two-thirds are aimed at Europe. NATO, which is scheduled to deploy 572 missiles, already has installed about 134 in West Germany, Italy, Britain and Belgium.

Tass disputed Washington's arithmetic, noting that U.S. leaders "deliberately ignore the American forward-based nuclear systems and also exclude from the count the British and French systems, as if they were nonexistent."

The Tass article ended on a mild note, however, saying the time had come "not to miss possibilities to improve Soviet-American relations and the international situation in general."

Given its conclusion, the Tass article indicated that Washington's rejection of the freeze proposal did not close the door to improved U.S.-Soviet relations.

Members of the U.S. congressional delegation said the Soviets offered no further details on Gorbachev's proposal than were included in the Pravda interview. They also said that Reagan's plans for a space-based defense system topped the list of Soviet concerns during sometimes heated debates on arms control.

The delegation, which also includes House Minority Leader Robert H. Michel (R-Ill.), hopes to meet with Soviet Foreign Minister Andrei Gromyko on Tuesday and with Gorbachev on Wednesday, although the meetings have not been confirmed.

The congressmen are expected to raise concerns about human rights and about the recent killing of U.S. Army Maj. Arthur D. Nicholson Jr., which they see as obstacles to improved relations. The Soviets have maintained that Nicholson was caught spying on a Soviet military installation in East Germany.

The congressmen, here on a parliamentary exchange, met today with deputies of the Supreme Soviet, or parliament, Soviet Minister of Foreign Trade Nikolai Patolichev and Agriculture Minister Valentin Mesyats.

According to members of the delegation, the Soviets expressed eagerness for improved trade relations but complained about a history of trade embargoes and sanctions, which they said had made business dealings with U.S. firms unreliable.