Soviet Communist Party chief Leonid Brezhnev accused the United States tonight of abandoning a previously "constructive approach" to nuclear arms control and adopting "a one-sided position" in the bargaining with Moscow.

While Brezhnev's remarks in a speech at a banquet honoring visiting Cuban leader Fidel Castro generally underscored Soviet opposition to Carter administration proposals on curbing strategic weapons, they were somewhat milder than earlier Kremlin comments. Indeed, there may even have been a hint that compromise suggestions would be considered.

"A reasonable accommodation is possible," Brezhnev said, "but it is necessary that not only we, but also the other side, should fully realize its responsibility for curbing the arms race and search for mutually acceptable solutions not in words but in deeds."

The possibility of moderation in Brezhnev's message, especially contrasted with the stridency of Foreign Minister Andrei Gromyko at a press conference the day after the Soviets rejected American proposals last Wednesday, will no doubt be probed by Washington. But for the moment, the dominant Soviet theme remains negative.

"Recent contracts and talks," Brezhnev said, "showed that instead of moving our partners (in SALT negotiations) are foregoing their constructive approach and keeping so far to a one-sided position."

Secretary of State Cyrus R. Vance offered the Soviets two options last week: ratification of the 1974 Vladivostok accord setting aside the issue of the U.S. Cruise missiles and the Soviet Backfire bomber that has blocked agreement thus far: or alternatively, "deep cuts" in existing nuclear arsenals.

President Carter and his advisers have portrayed a meeting next month between Vance and Gromyko in Geneva as crucial to the arms bargaining if an agreement is to be reached before the current one runs out in October. Brezhnev, however, did not mention any specific future meetings on SALT. Knowledge Western sources here say the Soviet have made no commitment to discuss the nuclear issue at the May session.

Members of Vance's delegation returning from Moscow told reporters accompanying them that there was an understanding that SALT will be discussed at Geneva even though no mention was made of this in the final communique.

Although Brezhnev's comment on SALT tended to overshadow the rest of his speech, the bulk was devoted to another major topic: relations with Cuban and Africa. Castro came to Moscow after an extended African tour, arriving here yesterday only hours after Soviet President Nikolai Podgorny, who had been visiting three key southern African states and Somalia.

Diplomats say it seems certain that Castro and the Soviet leadership are exchanging detailed assessments of the African situation, concentrating on their support for nationalist guerrillas in the fight against the white-minority regime in Rhodesia. The Cuban leader is also thought to have reported on his efforts to forge a pro-Moscow coalition between long-time adversaries, Somalia and Ethiopia.

But Brezhnev, in his banquet toast, denied that the purpose of the Soviet-Cuban alliance in Africa - which scored its greatest triumph last year backing the victorious Marxist forces in the Angolan civil war - was to impose Kremlin will on the continent.

"The opponents of Cuba, the opponents of socialism," he said, "are breeding the most absurd concotions about the aims and intentions of socialist countries in respect to these emergent states. But no slander can overturn facts."

In an answering toast, Castro picked up the theme of Soviet criticism of Carter for his support of human rights and asserted: "Those of us who have visited Africa and seen the traces of colonialism, capitalism, imperialism and racism well understand what human rights the imperialists are defending."

He said that his visit to Africa showed that the nations there "are making a final choice of the road towards socialism . . . that struggle is backed by our complete solidarity."

As a sidelight to the Castro visit, Brezhnev has been shown on television welcoming the Cuban leader and then meeting with him today. Compared with the grim pallor of his appearance during the talks with American last week, which was interpreted as evidence of his worsening health, Brezhnev's spirits seem to have been revived by the Cuban visit and he looked noticeably better.