Interior Minister Tomas Borge, a founder of the Sandinista National Liberation Front and considered the most powerful member of Nicaragua's new government, has been in Cuba since Monday on an undisclosed mission, authorities acknowledged today.

Borge's trip was not announced before he left and only officially was confirmed this morning after rumors about it began circulating in Managua. The visit is considered sensitive because of Borge's high position in the new government and suspicions, notably among some U.S. congressmen, that Nicaragua's revolutionary government may have closer ties to the Communist government in Cuba than previously claimed.

Borge is expected to return here either late tonight or tomorrow. His trip is the second visit to Cuba known to have been made by high-ranking Nicaraguan officials since the Sandinistas came to power July 19.

Junta members Moises Hassan and Daniel Ortega and Culture Minister Ernesto Cardenal were in Cuba the first week after the government of former president Anastasio Somoza fell in mid-July. Their trip was largely symbolic, a gesture of thanks by the new government here for the political and logistical support Cuba gave the Sandinistas during their years of battling the Somoza government.

The purpose of Borge's trip, however, may be more substantive. He is an old friend of Castro's and has visited Cuba several times before the Sandinistas succeeded in ousting Somoza. Borge is considered one of the most "radical" members of the Sandinista coalition that is governing Nicaragua.

Borge has said, however, on numerous occasions that Nicaragua will not be a "second Cuba" and has loyally and forcefully espoused the new government's generally moderate policies since it came to power.

As interior minister, a member of the Sandinista National Directorate and as one of three members of the high command of Nicaragua's embryonic army, Borge is believed to have a key role in determining Nicaragua's future policies.

Manuel Espinoza, the official spokesman for the junta, and Francisco Asis, one of Borge's top aides, both refused today to say why the interior minister was in Cuba or whom he would meet while there. Hassan would say only that Borge was in Havana "working for the Nicaraguan revolution."

A spokesman for the American embassy here said it had no information about the interior minister's trip and was not even sure when he had left. The spokesman added that he assumed Borge was in Havana "to renew old acquaintances."

Although both the Sandinistas and the Cubans have denied that Castro gave arms to the Nicaraguan guerrillas during their war with Somoza, Cuba has sent medical teams, journalists and substantial aid to Nicaragua since the fighting ended. Castro also has offered to send teachers here when Nicaragua's schools resume operation next month.

The new government's ties with Havana are in sharp contrast to Somoza's relationship with Castro. The old government here cut diplomatic relations with Cuba shortly after Castro came to power 20 years ago and Somoza allowed the U.S. Central Intelligence Agency to use Puerto Cabezas, on Nicaragua's Caribbean coast, to train Cuban exiles for the Bay of Pigs invasion in 1961.

Despite the warmth between Nicaragua and Cuba since the Sandinistas came to power. Government leaders here have gone out of their way to play down the relationship.

Hassan said, for example, after the restoration of diplomatic relations between the two countries last month that the move had no particular ideological significance.

"It doesn't mean we're rightists because we have an embassy in Chile," he said. "It doesn't mean we're rightists because we have an American embassy. [The restoration of ties with Cuba] doesn't mean anything either. It means we have a relationship with all countries in the world."