The Soviet Union accused the United States today of deliberately provoking a military confrontation with Libya and said the actions of the U.S. 6th Fleet had helped "poison the atmosphere" created at the summit in Geneva last November.

While it mounted a forceful campaign of words against U.S. military actions, however, Moscow seemed determined to play down its own role in a crisis that involves one of its oldest allies in the region. Vladimir Lomeiko, a spokesman for the Soviet Foreign Ministry, indicated at a press briefing that Moscow's reaction to U.S. firing on Libyan targets would be limited.

"The Soviet Union has provided moral and political support to the Libyan people and will take all measures it considers appropriate within the framework of existing treaties," said Lomeiko. He said the Soviet Union would urge other nations "to take steps to support the sovereign state of Libya."

Western diplomats here say the Soviet Union is eager to avoid any confrontation in the Mediterranean and in the Arab world, where it has been trying to build a reputation as a stable diplomatic partner.

In contrast, Soviet propaganda recently has played up charges of U.S. "neoglobalism," a term for intervention in the internal affairs of the other countries. Thus, Tass tonight said U.S. actions against Libya were prompted by Libya's "anti-imperialist, independent course."

Lomeiko characterized 6th Fleet actions as being "in the same spirit" as a U.S. decision last Saturday to proceed with a nuclear test in Nevada.

The test was carried out after Soviet leader Mikhail Gorbachev extended a unilateral Soviet moratorium on testing. Lomeika criticized that U.S. action today as a "provocation against the spirit of Geneva." The two actions served "to cast a shadow on all agreements reached," he said.

Prospects for the next summit, now waiting on an agreement on dates, depend on the creation of favorable conditions, he said.

Lomeiko sidestepped questions about Soviet military support for Libya and in particular about the role of Soviet advisors and Soviet-made SA5 missiles, which were delivered last December. Soviet military aid is "aimed exclusively at the sovereign defense" of Libya, Lomeiko said. He did not answer a question about possible Soviet casualties in Libya suffered during a U.S. strike reportedly made against a missile site.

Western diplomats here estimate that the Soviets have as many as 6,000 advisors in Libya, with as many as 2,000 working on an estimated seven SA5 missile sites. Syria is the only other non-Warsaw Pact country to which Moscow has agreed to provide the long-range, surface-to-air missiles.

Western diplomats here said it was unclear whether Soviets had been manning the SA5 missiles, which reportedly were fired on American warplanes. Traditionally, Soviets have insisted on control of such weaponry, but according to diplomats here, groups of Libyans have also received training on the SA5s in the last year.

Lomeiko said the Soviets were not conducting "any monitoring activity in the area" of the Gulf of Sidra, thus disputing U.S. claims that Soviet naval vessels have been within sight of the clashes. He also dismissed as irrelevant American arguments over Libya's claim to territorial rights in the Gulf of Sidra. Lomeiko said Libya's claims to the "historic bay" were similar to Britain's claim to the Gulf of Bristol and the United States' claims to the Chesapeake Bay.

The delivery of the SA5 missiles, first requested by Qaddafi in 1981 but granted only last year, is viewed by diplomats here as a key moment in Soviet-Libyan affairs. Qaddafi has been a close, but at times troublesome, ally for Moscow, largely because of his history of tense relations with his neighbors. He has never shared Moscow's Communist ideology and his own brand of radical politics, mixed with a strain of religion, has often made the Soviets nervous.

Still, Libya has been a major Soviet arms buyer, with purchases estimated at more than $20 billion in 1983. Since 1984, Libya, strapped for cash, has used its oil to pay its arms debts, estimated by one source at between $4 billion and $5 billion.

Qaddafi was given a warm welcome when he came to Moscow last October, which was seen as a sign that Moscow is interested in affirming its old alliances in the region, as well as seeking new ones.

However, Qaddafi's continued reluctance during the 1985 trip to sign a friendship and cooperation treaty -- the standard pact between Moscow and its close allies -- was taken to mean that the Soviet-Libyan relationship still has its difficulties.