The United States has given Nicaragua's radical government "suggestions on how our relations can be improved," but Nicaraguan sources said yesterday that the message was largely a reiteration of U.S. demands that Nicaragua halt its alleged aid to leftist guerrillas in El Salvador.

The State Department said only that the U.S. ambassador in Managua, Anthony Quainton, had delivered a message to the Sandinista-dominated government there last Friday offering suggestions on ways to improve relations. The department refused to give details.

However, Nicaraguan sources said the U.S. suggestion was a response to a detailed proposal made by Managua on May 7. In it, the Sandinista regime proposed high-level negotiations to discuss its ties with the Salvadoran guerrillas and other key points of tension with the Reagan administration.

According to these sources, the United States, after delaying a response for almost two months, again refused to address the question of moving to actual negotiations. Instead, the U.S. message repeated what the sources called "unsubstantiated allegations" of Nicaraguan interference in the Salvadoran civil war and said that a halt to such activities is a precondition to any improvement in U.S.-Nicaraguan relations.

The U.S. message marked the latest round in an exchange that began in early April when a senior State Department official summoned reporters to a special briefing and revealed that the administration, responding to pressures from Congress and other hemispheric governments such as Mexico, had offered Nicaragua an eight-point proposal as the basis for negotiations to improve relations.

A few days later, Nicaragua replied that it was willing to negotiate on these eight points and any other subjects of interest to the United States. It also called on Washington to fix a date for the start of talks.

Immediately afterward, however, senior administration officials privately told The Washington Post the United States intended to stall the start of negotiations because it believed the Nicaraguans were insincere and needed to be pressured further to stop aiding the Salvadoran insurgents.

Since then, in public statements both to the press and to Congress, State Department officials have denied stalling. But each subsequent Nicaraguan call to launch talks has been met here with lengthy delays, interspersed with statements that the United States is studying the Nicaraguan proposals or requesting clarifications.

The official U.S. position is that these exchanges represent a continuing dialogue that demonstrates Washington's good faith in seeking to relieve tensions with Nicaragua. In private, though, the U.S. attitude remains one of doubt that negotiations at this time would offer any real hope of achieving the administration's goal of cutting off the arms, guidance and other aid that Washington contends is flowing to the Salvadoran guerrillas from Nicaragua.

As a result, the administration believes that entering into highly visible negotiations would only give the Sandinistas an opportunity to win a propaganda advantage over Washington.

U.S. sources, noting the economic unrest in Nicaragua and the agitation of anti-Sandinista dissidents, are known to think that time is on Washington's side and that by allowing pressures to build, the Sandinistas gradually will be forced into making concessions.