SAN JOSE, COSTA RICA, JAN. 21 -- A Sandinista delegation issued new proposals today to soften the conditions for a cease-fire and to give Nicaraguan rebels "irrevocable guarantees" of political rights under international supervision after they lay down their arms.

The proposals were issued in a written statement by Nicaraguan Vice Foreign Minister Victor Hugo Tinoco after a day of intense maneuvering by both sides over the resumption of cease-fire negotiations.

Tinoco's delegation, which arrived here unexpectedly last night, called today for immediate direct talks with leaders of the Nicaraguan rebels, known as contras. Rebel leaders rejected the idea and issued a counterproposal for talks next week between working delegations of both sides.

The maneuvering came as each side accused the other of insincerity and duplicity in what was seen as a round of one-upmanship, leading up to crucial votes on U.S. aid to the rebels in Congress Feb. 3 and 4.

Tinoco and Sandinista military intelligence chief Maj. Ricardo Wheelock brought with them the same cease-fire plan that Nicaraguan President Daniel Ortega announced in Washington Nov. 13 and that has been rejected by the contras as a proposal for their "orderly surrender."

Later, however, Tinoco issued two new proposals "to enhance the prospects for achieving a cease-fire with the contras and incorporating them into the political life of the country with irrevocable guarantees that they will enjoy full political rights and civil liberties." His statement said the guarantees would include "the right to organize a new political party, or join existing parties, and participate fully in free and fair national and municipal elections."

Tinoco proposed establishment of "a special international commission to assure that these rights and liberties are fully enjoyed by all contras who elect to compete for power politically rather than militarily." He said the commission would be made up of representatives of the United Nations, the Organization of American States, Latin American countries and several international political groups, including the U.S. Republican and Democratic parties.

"These organizations would serve as guarantors that Nicaragua's commitment to full democratization," as set forth in a Central American peace plan, "will be fulfilled, and that all contras who renounce the use of military force will enjoy the benefits."

In addition, Tinoco said, the Nicaraguan government now is willing to allow the contras in agreed cease-fire zones to receive humanitarian aid from other governments, including the United States, after a cease-fire takes effect, provided the aid is delivered through international organizations, such as the Red Cross. By humanitarian aid, Tinoco said he meant food, clothing, shelter and medicine. He said Nicaragua continued to maintain a "strong objection" to assistance by governments in the absence of a cease-fire agreement.

The Sandinista delegation said it had decided to take advantage of a trip here by the Nicaraguan mediator in the truce talks, Cardinal Miguel Obando y Bravo, to make its surprise proposal for direct talks with rebel leaders. Six directors of the Nicaraguan Resistance, the political alliance of the rebels known as contras, flew in yesterday for a meeting with Obando to discuss their next moves in the negotiating process.

For their part, the contras were understood to have discussed with Obando new proposals under which additional U.S. aid would be held "in reserve" during a cease-fire. The proposals linked a cease-fire to a general amnesty and democratic reforms in Nicaragua, and the contras endorsed demands by opposition parties in the country for 17 constitutional changes.

In a news conference after meeting with Obando at a Roman Catholic school, the contra leaders said they had agreed to face-to-face talks with the Sandinistas here Jan. 28 and 29.

"The only meeting planned for today was our meeting with the cardinal," said Adolfo Calero, one of the six directors of the Nicaraguan Resistance.

Another director, Alfredo Cesar, said that a cease-fire must be reached in tandem with resolution of political problems, which the Sandinistas so far have refused to discuss with contra leaders.

Cesar said the Sandinista proposal for talks with the directors today was "completely unacceptable." He added, "They are still playing games." The contra leaders said that in any case, they would not deal with a delegation at Tinoco's level.

"If Daniel Ortega and his brother {Defense Minister Humberto Ortega} came here today, we could achieve positive results for peace," said contra director Aristides Sanchez.

Contra leaders charged that the Sandinistas were pulling out all stops to defeat new U.S. aid to the rebels. If the administration's aid request were defeated, they said, Sandinista willingness to make concessions would disappear.

"If the Sandinistas knew we didn't have any aid, they wouldn't have any reason to negotiate with us," said Pedro Joaquin Chamorro, another contra director.

In a news conference earlier, Tinoco repeated a Sandinista cease-fire proposal that would confine the rebels to three zones totaling 4,000 square miles in remote, sparsely populated areas in the north, center and southeast of the country.

In response, the contras have countered with a proposal for remaining in their "military operations zones" during a cease-fire and for sweeping military, political and economic changes that would, in effect, dismantle Sandinista control of the state. President Ortega has called that proposal absurd.

Accompanying Tinoco and Wheelock as part of the Sandinista negotiating team were American lawyer Paul Reichler and a West German legislator, Hans-Jurgen Wischnewski.

Tinoco repeatedly dodged questions on whether the Sandinistas would accept participation in the peace talks by members of the Nicaraguan internal opposition as demanded by the contras. He said domestic opponents who were arrested in Managua for having met with contra leaders in Guatemala this month had told Sandinista authorities they were duped into the meeting and wanted no part of such a negotiation.

Tinoco said he had received no formal proposal from the contras for inclusion of the internal opposition in the talks.