The United States has made progress toward establishing "a framework for mediation" of Nicaragua's internal strife, but full agreement on a workable mediation plan has not yet been reached, the State Department said yesterday.

The department's statement followed reports from Nicaragua that President Anastasio Somoza has agreed to accept "friendly cooperation" to help resolve the conflict that has caused hundreds of deaths in the past month.

Sources in the opposition groups that have been trying to loosen Somoza's grip on Nicaragua said in phone conversations from Nicaragua yesterday that there can be no basis for negotiation until Somoza agrees to their main demand - that he resign before his term expires in 1981.

Officials at the State Department indicated privately, however, that the talks being carried on in Nicaragua by a special U.S. envoy, William Jorden, have not yet focused on the question of whether Somoza will agree to step aside as a precondition for starting a mediation process.

One of the main problems, the officials said, is to prevent Somoza from trying to turn the mediation proposal to his advantage and use it as a device for strengthening his grip.

The United States, the officials noted, has indications that Somoza may be trying to stack the membership of any mediation team contributed by other hemispheric countries with representatives from the region's military dictatorships.

The officials also said Somoza has shown signs of being willing to negotiate only with selected parts of the broad opposition that has grown up to his rule in Nicaragua.

For that reason, the officials added, Jorden, whose talks with Somoza were continuing yesterday, has been instructed to stress that the mediation plan must inlcude the main opposition forces and make use of mediators from countries they trust.

To that end, the officials said the United States has suggested as possible participants in the mediation effort such democratic governments as Mexico, the Dominican Republic, Columbia, Venezuela, Jamaica, Trinidad-Tobago and Barbados.

If the contending parties wish, the Unite States also is willing to be mediator, the officials said, and would select a senior diplomat or another person with outstanding credentials as a negotiator.

A department spokesman, Kenneth Brown, underscored these point by saying: "Obviously, for the discussions to be successful, the framework and structure for the process will have to be acceptable to the opposition grups as well as the government of Nicaragua."

Brown said U.S. contacts with Somoza's domestic opponents have included the Broad Opposition Front, a coalition of political, business and civic groups, as well as separate consultations with businessmen, the opposition political parties and Catholic Church representatives.

Brown added that no contact had been made so far with factions of the Sandinista Liberation Front, a coalition of leftist guerrilla that have been fighting Somoza's forces. He said there was an understanding that information about U.S. contacts with the other opposition groups would be relayed by them to the Sandinistas.

Sources in the Broad Opposition Front contacted yesterday by phone said they were discussing the U.S. plan but had not yet been contacted directly by Jorden. Their contacts to date, they said, had been with the U.S. Embassy in Managua.

One opposition strategist warned that "no climate for negotiation" would exist until Somoza eases martial law and press censorship and releases the more than 300 opposition leaders under arrest since Aug. 25.

The sources continued to insist that the opposition has no interest in any plan that would allow Somoza to complete his term, although they said they were willing to negotiate the specifics of when and how he leaves office.

Should the more moderate opposition forces agree to Somoza staying on until 1981, even with restrictions on his power, they would risk the liklihood of a break with their militant Sandinista collaborators.

The U.S. effort to mediate is being pursued under the umbrella of a resolution approved Saturday night by the Organization of American States. It noted Nicaragua's willingness to accept the "friendly cooperation and conciliatory efforts" of outside countries.

About 1,500 persons have died in fighting during the past month. Somoza's military force, the National Guard, has successfully put down most of the armed resistance, but U.S. officials fear that, without a mediated solution, the violence is certain to flare up again.