The United States supports continued international mediation to resolve the Nicaraguan crisis despite President Anastasio Somoza's rejection of an American-Proposed referendum on his rule, the State Department said yesterday.

Spokesman Hodding Carter III commented guardedly on the situation, telling reporters the department had not yet seen a transcript of Somoza's remarks in a press conference Tuesday night nor received a formal rejection.

A U.S. compromise called for a national vote within 60 days on whether he should remain in power.

At the organization of American States, Costa Rica called for expulsion of Nicaragua because of a conflict between the two central American countries at their border Tuesday. Costa Rica broke diplomatic relations with Nicaragua.

In Managua, Somoza came under new pressure as a result of the Costa Rica action.

Members of the Broad Opposition Front, which had been negotiating with the president, ended their efforts Tuesday night when their deadline for Somoza's resignation passed with the 52-year-old West Point graduat still in office.

The Sandinista National Liberation Front, formed in 1961 to overthrow Somoza, has been training guerrillas in Costa Rica.

Nicaraguan government sources said the break in relations would stifle Central American commerce which normally moves across the Nicaraguan-Costa Rican border.

Costa Rica shut down the border crossing in Penas Blancas - stranding dozens of freight trucks on both sides - after a border clash that killed two Costa Rican guards and a Nicaraguan.

The violence with its southern neighbor and tension in the capital convinced many Nicaraguans that another civil war was brewing like the one in September that killed more than 1,500 persons, most of them civillians. Thousands of Nicaraguans have fled into Costa Rica.