The Soviet Union yesterday announced that "preliminary agreement on joint work" has been reached between the famed Kirov Ballet company of Leningrad, and George Balanchine, the Russian-born American choreographer who is generally acknowledged to be the most vital force in shaping modern vallet.

The announcement, carried in a brief news item by tass, the offical Soviet press agency, did not offer any more details. But informed Western cultural sources here in the Soviet capital said that any serious collaboration between Balanchine and Oleg Cinogradov, the Kirov's new director, could have considerable artistic impact within the U.S.S.R.

Balanchine and his New York City Ballet have toured the Soviet Union triumphantly on several occasions, delighting Soviet audiences with the choreographer's innovative concepts - which, more than any others, have set the tone of 20th century dance. They were a refreshing change in place of the steady diet of 19th-century classics and 20th-century political polemics that Soviet audiences are fed - and that have driven several important Soviet dancers to the West. The idea now of Balanchine bringing his ascetic, abstracted kind of dance to the Kirov would be not only a significant change in Soviet arts policy, it is also a case of Balanchine the man going full circle, because it was at the Kirov, then the Maryinsky company that his career began. He came West early and has made his base New York for the last 40 years.

But a spokesman for Balanchine said yesterday that it will not be in the nature of a bomecoming. So far, he said, there have been no formal approaches from the Soviets, only feelers from an old associate in the Russian dance world. Specific works have not been discussed. It is not anticipated that Balanchine would actually travel to Leningrad to stage the works - which would presumably come from among his existing creations. Balanchine would send somebody else to stage them.

Vinogradov, the new chief choreographer of the Kirov, clearly signaled his desire to modernize the repertoire of a company that has been buffeted by Western cultural influences. Three dancers who are now accorded superstar status in the West, Rudolf Nureyev, Natalia Makarova, and Mikhall Baryshnikov in 1974. In each case, the dancers said they left in search of artistic freedom they felt was absent in the Soviet Union. They have consistently objected to the use of the term "defector" with its heavy implication of political differences with Soviet rule.

Tass quoted Vinogradov as saying. "In our quests for a new language of the ballet, we shall base ourselves as before on the best traditions of Russian ballet . . . and will widen (the Kirov's) cooperation with the best choreographers in the West and add to its repertoire some contemporary Western works." Roland Petit, the French choreographer, had been invited to produce the ballet "Notre Dame Cathedral," tass said.

Russian dance companies have toured in the West and U.S. for many years, generally drawing critical acclaim for their superb technique and ensemblers. But estern critics have said that the Russian Ballet companies' repertoires have lagged because of the interference of cultural commisars who feel comfortable with the tried and true and dislike experimentation.

American dance companies touring here, such as the city center's Joffrey Ballet and the Alvin Ailey Dance Theater, with their mixture of contemporary themes, occaional use of jazz music, and advanced staging techniques, have astonished their Russian audiences. No Russian ballet company, no matter how modern and advanced some of is repertoires might be approaches the Americans in dance innovation. Even lightly publicized appearances by American companies generate packed halls here in a nation where many people follow ballet like American follow baseball.