SAN JOSE, COSTA RICA, JAN. 17 -- After having announced concessions here yesterday that kept alive a Central American peace plan, Nicaraguan President Daniel Ortega now faces the challenge of meeting those commitments to the satisfaction of his neighbors and the U.S. Congress, while maintaining the support of Sandinista Front militants at home, Central American officials said today.

A wide range of observers agreed that the Sandinista declaration near the end of a two-day regional summit meeting averted an immediate breakdown of the regional peace plan for which Costa Rican President Oscar Arias won the 1987 Nobel Peace Prize.

But now, Nicaraguan delegation sources acknowledged, the Sandinistas must move on their pledges within the next two weeks to increase their chances of achieving their main short-term objective: the defeat in the U.S. Congress of further aid to the rebels, also known as contras.

"We feel we have a better chance right now" of defeating contra aid, said a member of the Nicaraguan delegation. He added, however, that "the Congress is very, very conservative, and no one is figuring that the ball game is over."

Arias, on NBC's "Meet the Press," said "the future of more aid to the contras is entirely in Daniel Ortega's hands." If he "shows good faith" in carrying out his promises "then there's no more reason for aid to the contras because there's no more reason for more war," he said. But if Sandinista compliance with the peace accord turns out to be "cosmetic," the Reagan administration would probably "find support in the Congress for more aid to the contras."

Contra leaders called the Sandinista announcement "cosmetic" and "minimal" and reiterated demands for political negotiations dealing with what they say are the causes of the anti-Sandinista rebellion.

While most attention so far has focused on Ortega's acceptance of direct cease-fire talks between the Sandinistas and the contras, the most potentially far-reaching concession may be the lifting of a five-year-old state of emergency that curtails a number of civil rights, conference sources said.

A joint communique yesterday by Arias, Ortega and the presidents of El Salvador, Guatemala and Honduras called for "immediate" steps to comply with the peace accord they signed last August.

A one-page, written statement put out separately by Ortega as part of the deal with his colleagues announced that the Nicaraguan government would immediately suspend a state of emergency, open "direct talks" with the rebels on a cease-fire, free prisoners eligible under a partial amnesty and hold elections for local offices and a Central American Parliament.

Presidents Jose Napoleon Duarte of El Salvador and Jose Azcona of Honduras staunchly resisted a Nicaraguan proposal for a 30-day extension of the peace accord's compliance deadline, which ran out earlier this month. That would have set the deadline beyond the scheduled Feb. 3-4 voting in Congress on new aid for the rebels.

The Sandinista aim, one member of the Nicaraguan delegation said, was to get agreement on "a document that makes it very clear {to U.S. congressmen} that a vote for contra aid is a vote against the peace plan."

Although that effort failed and no deadline for compliance was specified, a de facto limit is the scheduled congressional vote, conference participants said.

A complicating factor for the Sandinistas is the certainty that the concessions announced by Ortega will now come under intense scrutiny from critics who argue there is less there than meets the eye. And the Sandinista leadership can be expected to provoke some questioning of the concessions as it tries to explain the elements less palatable to hard-line members of the Sandinista National Liberation Front.

A press conference after the signing ceremony yesterday suggested that Ortega had already started turning his attention to that problem. In reply to a question about his acceptance of face-to-face negotiations with the rebels on a cease-fire, Ortega said that two foreigners previously put forward to represent the Sandinistas in talks with the contras now would be "accompanied by a Nicaraguan."

In his written announcement, Ortega said that "the government has decided to include Nicaraguans as part of the team that will carry forward the cease-fire talks with the mediation of Cardinal Obando." Nicaragua's Roman Catholic primate, Miguel Obando y Bravo, served as a mediator in two fruitless attempts at truce negotiations in the Dominican Republic.

At the last session, the contras refused to meet with an American lawyer, Paul Reichler, and a left-leaning West German politician, Hans-Juergen Wischnewski, whom the Sandinistas had asked to represent them in "direct talks" with the contras.

Asked whether his concession would give "legitimacy" to the contras, Ortega stressed that any "political dialogue" with them would continue to be ruled out until they had abandoned their rebellion, taken amnesty and accepted the "civic option."

For their part, contra leaders dismissed the concessions as inadequate. Adolfo Calero, a director of the Nicaraguan Resistance, the contras' political alliance, said in Miami that the measures amounted to a smokescreen directed at Congress.

Contra spokesman Bosco Matamoros said the rebel leadership demands a "tripartite dialogue which involves the Sandinista party, the national civic opposition and the Nicaraguan Resistance." He said these talks should "deal with the roots of the Nicaraguan conflict."

On Jan. 15, the Nicaraguan Resistance accepted a call by a coalition of domestic opponents of the Sandinistas for direct negotiations among the Sandinista government, the contras and the "national civic opposition" in the presence of Cardinal Obando. A contra statement expressed support for "democratization, constitutional reforms and other political measures demanded by the national civic opposition."

The statement came after contra leaders had met in Guatemala Tuesday with 11 civic opposition representatives and agreed to call for such a "tripartite dialogue" to take place during the summit meeting.

In response, Sandinista authorities in Managua arrested four opposition members who had participated in the Guatemala meeting, charging them with involvement in a "terrorist conspiracy" to form an internal front with the contras.

Two more opposition leaders were arrested, one Saturday night and another Sunday, according to members of the detainees' families reached by telephone in Managua. The government there said 11 such leaders were under suspicion of taking part in a CIA-directed plot in conjunction with the contras.

The move suggested that Sandinista leaders will go to great lengths to head off any joining of forces by the contras and unarmed domestic opponents. Such a development, Sandinista leaders apparently believe, could give internal political overtones to any talks with the contras, undermining the Sandinista Front's repeated pledges to its followers never to place their revolution on the bargaining table.

In Managua late yesterday, key Sandinista leader Bayardo Arce struck a sharply pessimistic note about the prospects of the peace plan. Speaking to party militants, he warned, "If the accords fail, we must all be clear that the only course will be war and our policy will be: everything to fight the war." He added that political pluralism and private enterprise would be restricted "according to the requirements of the survival of the Nicaraguan revolution" if the peace pact fails and the United States gives new aid to the contras.

The contras also appear to be in a tight spot. They have no wish to enter negotiations that might help the Sandinistas look good to the U.S. Congress, while failing to meet contra demands for fundamental political changes. On the other hand, the contras cannot afford to appear intransigent in view of the upcoming aid vote.

The immediate test of Sandinista sincerity about their concessions may be the lifting of the state of emergency. It suspends constitutional rights of habeas corpus and several due-process provisions, provides for trials by special courts, restricts freedoms of expression and assembly and gives security forces leeway in acting against suspected contras.

Previously, the Sandinistas had insisted they would lift the state of emergency and implement an amnesty law only when a verification commission had ascertained that Honduras had removed contra bases and U.S. aid to the rebels had ended.

Julia Preston of The Washington Post Foreign Service contributed to this report in Washington