SANTO DOMINGO, DOMINICAN REPUBLIC, NOV. 3 -- The first formal talks aimed at a cease-fire in the Nicaraguan war opened here today under the mediation of Nicaragua's Roman Catholic primate.

The mediator, Cardinal Miguel Obando y Bravo, met for 2 1/2 hours with a four-man rebel negotiating team at the headquarters of the Catholic archdiocese here. He was to hold talks with a Sandinista government delegation Friday, then meet again with the rebels, known as contras.

Obando said he brought specific suggestions for both sides on how to reconcile their widely divergent truce proposals. After meeting with the rebels, he said without offering further details, "There is a certain flexibility. I think we have to keep working with optimism."

The cardinal chose Santo Domingo, capital of the Dominican Republic, as a compromise site for the indirect negotiations to end a long dispute between the Sandinistas, who wanted the talks in Washington, and the rebels, who insisted that they be held in Central America.

{Church officials have tried to arrange direct talks, but none are planned in this session because of objections by the Nicaraguan government. Face-to-face negotiations are "very difficult for the time being," Obando said today, The Associated Press reported.}

U.S. Army Col. Richard Schaden, who is assigned by the State Department to advise the contras, arrived on the same flight with the rebel delegation and was later seen at the same hotel.

Bosco Matamoros, a spokesman for the rebel delegation, said Schaden "is not part of our group," adding, "I don't know what he is doing here." Schaden could not be reached for comment.

The Sandinistas' truce proposal, made three weeks ago, calls on the contras to surrender, accept amnesty and return to civilian life. The rebels insist on major political changes in Nicaragua before laying down their weapons.

Obando said before leaving Managua for Santo Domingo that he would seek at least a temporary truce by Christmas, even if major issues remain unsettled. The talks are a result of a Central American peace accord.

Through six years of fighting, until last month, the Sandinista government had rejected any talks with the contras and insisted instead that peace be negotiated with the United States, which finances the rebel insurgency.

As he dispatched a four-man delegation to the talks here, Nicaraguan President Daniel Ortega continued to make an issue of the rebels' ties to Washington and accused the Reagan administration of seeking to continue the war. "Our first impression {of the rebels' truce plan}," Ortega told reporters in Managua "is that this is a document prepared by the most right-wing sectors of the Reagan administration, intended to kill the chances for agreement on a cease-fire."

Ortega described the contra proposal as a maneuver to demand new political concessions in addition to those Nicaragua accepted when it signed the five-nation peace plan last August.

Rebel leaders denied that U.S. officials helped draft their proposal. "It would be just as ridiculous for us to say that every move by the Sandinistas is dictated by Moscow," said contra spokesman Matamoros.

The rebels' truce conditions include abolition of Sandinista party control of the news media, state collective farms, military conscription, party-controlled neighborhood vigilante groups and food rationing.