SAN JOSE, COSTA RICA, JAN. 14 -- President Oscar Arias today acknowledged that his Central American neighbors have failed to comply with a regional peace plan he initiated, and he warned that time is running out for an alternative to continued war in the area.

On the eve of a Central American summit meeting to assess compliance with the peace accord, Arias blamed a lack of "political will, tolerance and flexibility" for failure to end guerrilla wars in Nicaragua, El Salvador and Guatemala. He said his immediate aims at the summit would be to restart cease-fire talks between the warring parties and obtain further steps toward "democratization" from the Sandinista government in Managua.

"I want to persuade my colleagues that we must act now," Arias told a news conference. "There is not a single minute more to lose."

Arias and the presidents of Nicaragua, El Salvador, Honduras and Guatemala are scheduled to meet here Friday to evaluate the peace agreement they signed last Aug. 7 in Guatemala. They are to receive a report from the International Commission for Verification and Follow-up, a body formed to monitor the signatories' compliance with the peace plan's provisions for amnesties, cease-fires, democratization, cessation of aid to rebel forces and ending of the use of one state's territory to attack or destabilize another.

Arias' comments came amid indications that pressure is building on the Sandinistas to make major new moves that would give impetus to the peace plan before the U.S. Congress votes on renewed funding for the Nicaraguan rebels, known as contras, early next month.

Sen. Christopher J. Dodd (D-Conn.) and other opponents of contra aid have told Sandinista leaders in recent days that a stalemated peace process will strengthen the hand of the Reagan administration in seeking a major new aid package for the rebels.

Dodd said in an interview here today that during a visit to Managua this week he had detected a softening of the Sandinistas' longstanding refusal to hold face-to-face negotiations with contra leaders. But he said that "more substantive, irreversible measures," such as a broader amnesty, were needed from the Sandinistas to help him and like-minded congressmen resist new funding for the rebels.

A U.S. senator critical of the Sandinistas, Phil Gramm (R-Tex.), said here today that "everyone realizes that the Sandinistas have made a mockery of this agreement." He said a letter sent to Nicaraguan President Daniel Ortega by Arias yesterday clearly laid out Sandinista "noncompliance" with the peace accord.

In his news conference, Arias said, "I believe that we all realize that the alternative to the peace plan in Central America if we fail is the continuation of war. This is the last opportunity if we want to comply, because there are many people interested in burying this plan so that war will be the only option."

He did not say who he believed was trying to scuttle the peace plan.

Arias added later, "If we had complied already, there would be no more war in the region and the superpowers would not be thinking of more aid to the rebels. In a way, it has been our fault. There hasn't been the political will to comply. We have found excuses not to comply. If Nicaragua had complied . . . no one would be thinking of more aid to the contras."

In reply to questions, Arias denied that the U.S. national security adviser, Lt. Gen. Colin Powell, had suggested to him during a recent visit here that U.S. aid to Costa Rica could be curtailed if new funding for the contras fell through because of the peace plan.

"At no time have I been the victim of pressures from anybody," Arias said.

According to Dodd, however, Arias is "getting very angry" about suggestions from Washington that the deadline for implementation of the peace plan's provisions is up and that no more postponements should be tolerated.

"He's not in favor of long extensions, but he's not a quitter either," said Dodd, who conferred with Arias last night. Dodd said there appeared to be a consensus for extending the deadline for full compliance with the peace accord, as long as this involved "a very clear, very brief and very specific timetable."

The peace plan called for the signatories to reach full compliance simultaneously by Nov. 5, but that deadline was postponed until early this month.

So far the main achievements under the peace accord have been truce talks between the governments of El Salvador and Guatemala and their rebel forces; indirect contacts between Nicaragua and the contras; a new amnesty in El Salvador; the release of some political prisoners in Nicaragua and a loosening of restrictions by the Sandinistas on their domestic opponents.

However, no lasting cease-fires have been negotiated, and there appears to be no prospect in sight for ending the wars that have devastated the region's economies, caused massive flights of refugees and taken tens of thousands of lives this decade.

In a new report on Sandinista compliance with the human rights and democratization provisions of the peace accord, the Washington-based Puebla Institute said today that "the Nicaraguan government, in the main, has failed to meet its human rights commitments, taking only a small number of the measures needed to fulfill the accords dealing with civil and political rights."

The Puebla Institute, a conservative group that describes itself as a "lay Roman Catholic human rights organization," is headed by Humberto Belli, an exiled Nicaraguan opponent of the Sandinista government.

A 102-page report released today by the institute charged that as of this month, the Nicaraguan government had "partially implemented only six of the 58 steps identified by the Puebla Institute as essential for democracy."

It said the measures taken included the reopening of the newspaper La Prensa and the church radio station, Radio Catolica; the return of three exiled priests; the release of some political prisoners and permission for some opposition demonstrations.

The institute said none of the reforms "resulted from institutional change," but could be rescinded at any time and appeared to be "largely good will gestures."

It charged that the human rights situation had even worsened in two areas since the peace plan was signed, with the reemergence of Sandinista mobs and increased sentencing by political courts called People's Anti-Somocista Tribunals. In addition, the report said, "Prisoners continue to be held in intolerable conditions . . . and to be subjected to various other types of torture and ill-treatment."