SAN JOSE, COSTA RICA, FEB. 5 -- Nicaraguan rebel leader Alfonso Robelo, acceding to an ultimatum from Costa Rican President Oscar Arias, announced today that he will resign as one of six directors of the rebels' political alliance.

Robelo said that to continue living in Costa Rica he would give up his post on the directorate of the Nicaraguan Resistance when the umbrella organization's assembly meets around Feb. 20 and chooses a replacement. But he said he would "continue working in the resistance in strictly political activities."

Robelo made the announcement in a letter he released today in response to a demand by Arias that three Nicaraguan rebel directors living in exile in Costa Rica either give up their seats on the directorate or move out of this country.

{The two other directors living in Costa Rica, Pedro Joaquin Chamorro and Alfredo Cesar, have announced they will move to Miami, where the contras maintain their principal office, The Associated Press reported.}

Arias issued the ultimatum Jan. 13, two days before a Central American summit meeting here, to bring Costa Rica into strict compliance with the regional peace plan for which he won the Nobel Peace Prize.

In an interview today, Robelo indicated that the move was not related to the rejection Wednesday by the U.S. House of Representatives of a new aid package for the rebels, known as contras. He said he wanted to stay because his wife, who is about to give birth, is Costa Rican and because he has business interests here.

He said he has "never received any salary from any organization" for his work as a contra director and that "I want to keep this independence."

In the interview, Robelo expressed deep pessimism about the contras' fate after U.S. authorization to resupply them runs out at the end of this month. He said the defeat of the Reagan administration's $36.2 million aid package would deal "a hard blow" to the contras militarily inside Nicaragua and seriously weaken the rebels' negotiating position in cease-fire talks with the Sandinistas scheduled for Feb. 10-12 in Guatemala.

The presidents of Nicaragua, El Salvador, Honduras, Guatemala and Costa Rica met here Jan. 15 and 16 and agreed to comply immediately with requirements of the peace accord that had not already been implemented. The accord, designed to end years of bloodshed in Nicaragua, El Salvador and Guatemala, was signed by the five presidents in August. It originally set a deadline of Nov. 5, 1987, for the simultaneous implementation of amnesties, cease-fires, democratization measures, an end to outside aid for insurgencies and a ban on rebels' use of one state's territory to attack another.

Since the last summit here, Robelo charged, three weeks have passed and Nicaragua still has not fully met its obligations to release political prisoners, stop aiding leftist guerrillas in El Salvador and guarantee civil and political rights.

Robelo, a former ally of the Sandinista National Liberation Front, was a member of a governing junta that took over in Nicaragua in July 1979 after the overthrow of dictator Anastasio Somoza. He formally broke with the Sandinistas in 1980.