The Soviet Union, accusing the United States of a "frenzied campaign of threats" against Nicaragua, carefully sidestepped questions today on what it would do in the event of a U.S. invasion.

In a prepared statement, Foreign Ministry spokesman Vladimir Lomeiko said the United States had fabricated reports of Soviet combat planes arriving in Nicaragua to justify an "unprecedented escalation of U.S. military presence in Central America." He said U.S. actions constituted a "gross violation" of Nicaragua's sovereign rights and of international law.

But Lomeiko, answering reporters' questions, avoided any warning of Soviet retaliation should its allies in Managua come under attack. Lomeiko also refused to discuss Moscow's military assistance to Nicaragua. Asked whether he could say what sort of military hardware the Soviet Union has been sending to Nicaragua, Lomeiko said:

"In the field of defense, it is an internal matter for the Nicaraguan government. Only the Nicaraguan government, like the government of any sovereign country that is subjected to persistent attacks, direct or indirect aggression, . . . can judge for itself the level necessary for the defense of sovereignty."

He turned away another question on Soviet intentions by saying the focus should be on preventing a crisis. Today's press briefing was called as the Soviet media continued a steady stream of news about the situation in Nicaragua, stressing the threat of an invasion by U.S. forces.

Since the Sandinistas came to power, the Soviet Union has given them full rhetorical support, economic aid and some military equipment -- helicopters, heavy cannons, civil aircraft and trucks through Bulgaria, diplomats here say. But the Soviets have resisted Sandinista appeals for more sophisticated weaponry, apparently out of concern for U.S. reaction.

"They have held the Sandinistas at arm's length. They have not given as much as the Nicaraguans would like, and what they have done has been done in a way not to antagonize the U.S. any more than necessary," one diplomat said.

Lomeiko said, "As in Grenada, we mustn't even admit a shade of doubt as far as our position is concerned. Everything must be done to prevent the development of such a situation.