MANAGUA, NICARAGUA, DEC. 16 -- President Daniel Ortega today accepted a proposal for new, more complex indirect negotiations between his Sandinista government and Nicaraguan rebels, and agreed to observe a Christmas truce.

After conferring this morning with Nicaragua's Roman Catholic primate, Cardinal Miguel Obando y Bravo, Ortega told reporters that the Sandinista side was ready to hold a second round of indirect cease-fire talks Monday with the participation of as yet unnamed foreign advisers.

Officials of the rebels, who are known as contras, said their side agreed in principle to the meeting in the Dominican Republic, but did not know yet whether they could assemble their negotiating team there by Monday.

A second round of talks, following an inconclusive first meeting Dec. 3-4, was to have started two days ago but was called off by the Sandinistas at the last minute.

Today's proposal, as explained by Ortega, appeared to mark a sharp variation on a long-espoused Sandinista theme that Managua should negotiate a peace agreement with the Reagan administration, the contras' "masters."

Now the Sandinistas seem to be trying to involve Americans as advisers on their side, with the apparent aim of demonstrating to the U.S. Congress that they are sincere about truce talks in view of an upcoming vote on aid to the contras, observers here said.

Alfredo Cesar, one of six civilian directors of the Nicaraguan Resistance, the contras' political alliance, said:

"If the Sandinistas come with Soviet and Cuban advisers -- the people who give the orders in Nicaragua -- I would think that would be very advantageous. But if they are from some other, western country, it doesn't make any sense." He added, "We have no foreign advisers, and we don't need any."

Obando said he had suggested that the Sandinistas and the contras name teams of "technical advisers," who would then meet face-to-face to iron out details of a cease-fire.

The Sandinistas in recent days have been in touch with the office of Speaker of the House Jim Wright (D-Tex.) in an effort to involve him or his representatives in the cease-fire talks in some capacity, congressional sources said.

"It appears there will be teams that both advise the cardinal and represent each side to him," an aide to Wright said. "Our position is we're willing to help the cardinal, but these advisers appear to represent both the cardinal and a party, and this is inconsistent with any role considered by the speaker."

Another American mentioned by a Sandinista official today as a prospective adviser, former U.S. arms control negotiator Paul C. Warnke, also said he was not interested.

"Nicaragua will have a team of foreign advisers," Ortega said in explaining the proposed negotiating arrangement. He declined to specify who these advisers would be, and other officials indicated they had not been chosen yet.

Ortega said the contras also would have a team of advisers, and that the two teams would meet face-to-face, with Obando participating as a mediator. He said he did not know what nationality the contra technical advisers would be, but it appeared that there was no Sandinista objection to their being Nicaraguan contras.

In addition to the technical team, Ortega said, the Sandinista side would send its previous negotiating team headed by Vice Foreign Minister Victor Hugo Tinoco.

"The Nicaraguan delegation will be in permanent communication with the team of advisers," Ortega said. But, as before, "no Nicaraguan member" of the Sandinista side would participate in direct talks with the contras. He said the advisers' role would be "only to deal with technical matters in order to reach agreement on a cease-fire."

Ortega said that as a gesture of appreciation to the cardinal, "We are going to decree a cease-fire for Dec. 24 and 25."

Contra spokeswoman Marta Sacasa said in Miami that the rebels already had proposed a Christmas truce. "That's the truce we proposed," she said when asked about Ortega's statement. "We're not accepting their truce. They're accepting ours." The statements suggested the Christmas truce would amount to each side's observing a unilateral cease-fire at the same time.

Washington Post correspondent Julia Preston contributed to this story from Miami.