President Anastasio Somoza's rejection of a U.S. mediation proposal to resolve Nicaragua's crisis could affect "the whole gamut of [U.S.] relationships," including trade and military agreements, with his government, a senior U.S. administration official said yesterday.

The official's remarks came in a briefing to reporters that apparently was designed to publicly convey strong U.S. displeasure at what he described as "stonewalling" by Somoza and his delegates to the threemonth-old talks.

The official, who refused to be directly identified but who is close to the mediation talks, also refused specifically to identify what steps the United States might take to pressure Somoza. He noted, however, that the United States conducts significant commercial trade with Nicaragua.

"WE DON'T HAVE MILITARY ASSISTANCE THERE" SINCE IT WAS CUT OFF LAST YEAR, THE OFFICIAL SAID, "BUT WE DO HAVE A MILITARY MISSION THERE" AS WELL AS AN AID PROGRAM WITH $23 MILLION OF APPROVED FUNDS STILL IN THE PIPELINE.

"WE'VE THOUGHT OF A LOT OF THINGS," THE OFFICIAL SAID. "I THINK SOMOZA'S RELATIONSHIPS" WITH THE UNITED STATES "MEAN A LOT TO HIM."

EARLIER YESTERDAY, SOMOZA OFFICIALLY TURNED DOWN A "FINAL" PLAN OFFERED BY THE U.S.-led mediators as "interventionist." The plan called for a nationwide referendum on his presidency to be organized and controlled by the Organization of Amercian States.

The talks between Somoza and a widespread political, business and labor coalition seeking his resignation began in early October following a cease fire in a civil war between the Nicaraguan National Guard and civilians led by Sandinista Liberation Front guerrillas.

The mediation, conducted by an international team including the United States, the Dominican Republic and Guatemala, has encountered a number of snags and has come close to breaking down on several occasions.

In the past, however, the United States has consistently refrained from publicly commenting on the stands of either Somoza or the opposition. Despite traditionally strong U.S. influence on Nicaraguan affairs, the United States has sought, as a member of the ostensibly neutral mediating team, to maintain a publicly non-partisan image in the conflict.

The U.S. official described the talks yesterday, however, as "at a critical point" and at "something of an impasse" due to Somoza's refusal to accept the latest mediation plan.

He noted that the opposition had completely accepted the proposal. He said that international organization and control of the planned referendum was viewed as necessary to convince voters that the election would not be fraudulent and they would not be punished for voting against the government.

Somoza's rejected of the condition as a violation of Nicaraguan sovereignty, the senior official said, is based on a concern that "under those, conditions, he might not win."

Throughout the mediation, the official said, Somoza has yielded only on those points "that have never touched the issue of his power," such as amnesty for political prisoners and the lifting of martial law.

Somoza, in a news conference yesterday, presented a counterproposal to the mediaters' plan. His offer called for joint government and opposition control of the election, but chances for its acceptance either by the mediation team or the opposition appeared dim.

Informed sources in Managua said yesterday that U.S. mediator William G. Bowdler had already indicated to Somoza that the plan was unaccaptable. Bowdler returned to Nicaragua yesterday morning following his urgent recall to Washington, along with U.S. Ambassador Mauricio Solam and U.S. Army Southern Command chief Lt. Gen. Dennis C. McAuliffe for consultations over the Christmas holidays.

Meanwhile, Somoza yesterday made his own bid at raising the public stakes in the increasingly tense Nicaraguan situation by saying an interview that he would not hesitate to invade neighboring Costa Rica "if they [Costa Ricans] warrant it."

Somoza has accused the Costa Rican government of harboring Sandinista guerrillas and allowing them to launch attacks on the National Guard across its borders.

U.S. concern that not only Costa Rica, but also the anti-Somoza governments of Venezuela, Panama and Cuba, might become involved in the conflict on the side of the Sandinistas has led to U.S. diplomatic representations to all of those countries in recent weeks, the senior official said.