SAN JOSE, COSTA RICA, JAN. 22 -- Nicaraguan rebel leaders meeting here are considering asking the Reagan administration to hold new lethal aid for their forces "in reserve" while cease-fire negotiations with the Sandinista government take place, rebel officials said today.

The officials insist that U.S. congressional approval of new funding for the rebels, who are known as contras, is necessary to make the Sandinistas negotiate.

However, in an apparent move to make voting for additional aid palatable to undecided congressmen following new Sandinista overtures, the contras indicated a willingness to have the lethal portion of such aid withheld for a set negotiating period. Contra leaders made a similar proposal when they met with President Reagan in Los Angeles in August.

The idea was among several proposals discussed yesterday in a meeting between the six directors of the Nicaraguan Resistance, the contras' political alliance, and the mediator in negotiations with the Sandinistas, Cardinal Miguel Obando y Bravo.

After that meeting, the head of a Sandinista negotiating team, Vice Foreign Minister Victor Hugo Tinoco, announced two new proposals to soften somewhat the conditions for a cease-fire and give "irrevocable guarantees" -- under international supervision -- to the contras for the full exercise of political and civil rights.

Contra directors expressed skepticism about the new Sandinista offers, which include a proposal for an international commission to ensure that the contras' rights are fully respected after they lay down their arms and "join the political life of the country." The commission would consist of representatives of a variety of international institutions and political parties, including the U.S. Republican and Democratic parties.

Contra director Alfredo Cesar told reporters that if the Sandinistas intended the returning contras to have the same political rights now accorded to the internal opposition, the offer would be meaningless.

Contra officials here strongly denounced Sandinista attacks against the domestic opposition today and last night.

"What guarantees can we have when Tinoco is announcing a political opening and, at the same time, the Sandinistas in Managua are using mob terror as their language of political dialogue?" asked contra spokesman Bosco Matamoros.

A statement prepared by the contras for their meeting yesterday with Obando proposed a "direct tripartite dialogue" involving the Sandinistas, the contras and the Nicaraguan internal opposition. This dialogue would simultaneously deal with a cease-fire and "solutions to the causes of the war, through the adoption of democratizing measures," the statement said.

While these talks were going on, it said, "the Nicaraguan Resistance would ask that new lethal aid, once granted, be placed in reserve for a period of 30 days." That period would correspond to a time frame for the negotiations with the Sandinistas to achieve acceptable results.

This and other proposals were understood to be still under discussion by the contra leadership, which is currently debating its approach to cease-fire negotiations scheduled here for Jan. 28 and 29. Contra leaders yesterday rejected a surprise call by the Sandinistas for immediate talks.

A contra official said a request to hold the lethal aid in reserve would be "an expression of our good will" for effective cease-fire talks, but he stressed that congressional approval of the aid was vital to guarantee Sandinista compliance with their commitments under a Central American peace accord.

"New lethal aid has to be there, not only to bring the Sandinistas to the negotiating table, but to keep them at the negotiating table," the official said.

The Sandinistas have shown that they strongly oppose any involvement in the cease-fire talks by the Nicaraguan internal opposition and any inclusion in the discussions of political matters relating to the restructuring of the government, Army or society in general.

Meanwhile, a Miskito Indian leader, Brooklyn Rivera, has announced plans to return to Managua Saturday to negotiate a settlement of autonomy demands with Sandinista authorities. Rivera says he commands about 2,400 Miskito Indian fighters, but other Miskito leaders have said this claim is exaggerated, and his group reportedly has not been active in fighting the Sandinistas for some time.

During a Central American summit meeting here last week, Rivera conferred with Nicaraguan President Daniel Ortega over breakfast and expressed willingness to return to Nicaragua.

Other self-styled Miskito "commanders" here reacted bitterly, taking out a newspaper advertisement to denounce Rivera.