The United States is continuing to urge the Soviet Union to support action to head off expanding warfare between Ethiopia and Somalia, a state Department spokesman said yesterday.

Secretary of State Cyrus R. Vance discussed the conflict with Soviet Ambassador Anatoliy F. Dobrynin last Saturday, it was learned. President Carter on Thursday accused the Soviet Union of "unwarranted involvement in Africa," and the Sovient Union rejected the charge.

Soviet aircraft have made about 50 flights into Ethiopia since mid-November, carrying weapons and personnel, State Department spokesman John H. Trattner said yesterday. About twothirds of the flights were medium and heavy transport planes, Trattner said, and the other were Aeroflot passenger aircraft.

Somalia charged yesterday that Soviet Defense Minister Dmitri Ustinov and Cuban defense chief Raul Castro are both in Ethiopia now, secretly planning an "air, land and sea" offensive against Somalia.

According to Somalia's minister of information, the attack on Somalia "is being planned by a high military committee comprising eight Russians, four Abyssinians (Ethiopians) and three Cubans."

American intelligence sources, who reported last week that Raul Castro was in Ethiopia, said yesterday that they had no knowledge that the Soviet defense minister is also there. U.S. specialists were skeptical of that portion of the report involving Ustinov. They said they assumed lower-ranking Soviet officials were with the estimated 3,000 Cuban and Soviet military advisers believed to be in Ethiopia.

The Soviet Union, through its official news agency, Tass, last Friday accused President Carter of "deliberate distortion of the true state of affairs" in the Ethiopian-Somalia conflict." Tass charged that Middle Eastern allies of the United States were secretly delivering American weapons to Somalia, the "aggressor" in the fighting. The State Department said the U.S. arms supply charge was "false."