The Carter administration, despite threats to "reassess" its relationship with Nicaraguan President Anastasio Somoza following his refusal to resign, has decided not to break diplomatic relations or recall the U.S. ambassador there.

Instead, informed sources said yesterday, the pressures applied against Somoza will be limited to such lesser steps as withdrawing the four-man U.S. military mission, reducing the size of the U.S. embassy staff or making further cuts in the already tiny amount of economic aid given to Nicaragua.

According to the sources, the State Department recently sent to the White House a list of the limited diplomatic actions it recommends taking in the wake of Somoza's refusal to accept a U.S.-sponsored mediation plan for resolving the Nicaraguan crisis.

The sources said that, within the next few days, President Carter is expected to choose one, or, more likely, a combination of these recommendations.

However, the approach decided on seems certain to be criticized as an ineffective slap on the wrist by liberal governments in Latin America and U.S. human-rights organizations.

Both forces have been urging the administration to use all the diplomatic weapons at its disposal to force Somoza to stand aside and allow free elections in his tiny Central American country.

Last September, resentments against the Somoza family's long rule erupted into three weeks of civil war between his National Guard troops and civilians led by the Sandinista Liberation Front guerrillas.

The sources said that as recently as last week the possibility of recalling U.S. Ambassador Mauricio Solaun from Nicaragua -- a step just short of breaking relations -- was under active and serious consideration within the administration.

But, the sources added, that option was discarded, partly because of concern over the reaction it would provoke among Somoza's supporters in Congress. These forces, led by Rep. Charles Wilson (D-Tex.), have threatened to retaliate by holding up legislation to implement the Panama Canal treaties and by seeking to cut U.S. aid to other countries charged with human-rights violations.

In addition, the sources said, the administration concluded that, since its relations with Somoza already are so strained, recalling Solaun would be ineffective and counterproductive.

The sources pointed out that the United States, which already has cut off its military aid to Somoza and which has made clear its belief that he should step down, doesn't really have much leverage left to use against him.

Yet, they added, the loss of U.S. backing didn't stop Somoza from reestablishing much of his old grip over Nicaragua after the September fighting. As a result, the sources said, administration officials reasoned that recalling the ambassador could prove a gesture that would only make him more resistant to mediation and negotiation.

Another factor, the sources said, is continuing U.S. concern that violent overthrow of Somoza would lead to control by Sandanista extremists and a possible communist government.

For these reasons, the sources continued, the administration decided that the best hope of keeping alive chances for a gradual transition to Somoza's more moderate opponents rests with keeping an ambassadorial presence with sufficient authority to maintain contact and negotiations with both sides.

The aim of the recommendations given the president, the sources said, is to lay out a course that will allow the United States to keep its hand in the Nicaragua situation, while demonstrating sufficient displeasure with Somoza to retain some credibility and influence with the moderate opposition.

In addition, they added, Washington also believes that future efforts to deal with the Nicaragua situation must continue to be cloaked in the mantle of hemispheric joint action rather than seen as a U.S. intervention in the affairs of a small country.

The unsuccessful mediation effort, although dominated by special U.S. Ambassador William G. Bowdler, was carefully set up under the authority of the Organization of American States. The administration, the sources said, is trying to gain support for convening another special meeting of OAS foreign ministers to consider new steps for dealing with the Nicaragua crisis.