President Fernando Belaunde Terry has received replies from Argentina and Britain to his unspecified 11th-hour peace initiative launched last night, but today's fighting on the Falkland Islands appeared to have doused the plan with frigid South Atlantic seawater.

Informed observers here saw the Belaunde plan, the third the Peruvian leader has proposed in the past 18 days and the first without the imprimatur of the United States, as a last-ditch Peruvian effort to avoid having to send military hardware, and possibly troops, to support the Argentine effort.

War fever is mounting here, as hard-liners within the Peruvian Cabinet and armed forces reportedly have been pressing Belaunde to come to Argentina's aid. Chief among them is the war minister, Gen. Luis Cisneros, born in Argentina, educated by its military and nicknamed "the gaucho."

As Belaunde attempted yesterday to promote the plan he called "a new and simple formula . . . which would open up a new road to encounter a solution," Cisneros said that British "aggression" impelled Latin America to respond under the terms set down under the Inter-American treaty of Mutual Assistance--the Rio Treaty.

Although most Latin American countries have offered strong diplomatic support of Argentina throughout the Falklands crisis, Peru has the closest relations with Argentina and is seen as the most likely to lend military support.

In an interview with the influential newsmagazine Caretas earlier this week, Gen. Cisneros said Peru could send Argentina "surface ships, submarines, airplanes, helicopters, ammunition, tanks, antiaircraft weapons, antitank weapons, personnel and all that country would need and Peru is in condition to send." Under military rule from 1968 to 1979, Peru acquired a hefty arsenal of U.S. and Soviet weaponry.

Belaunde said that his most recent proposal was made after he received the British Ambassador Charles Wallace, who told him of British efforts in the United Nations. In its response to the proposal today, Peruvian officials said, Britain conditioned its acceptance on Argentina's agreement on two crucial points Buenos Aires had rejected earlier: withdrawal of its troops and acceptance of Security Council Resolution 502. They said the Argentine response had been transmitted in a telephone call.

Neither the specifics of the new proposal nor the substance of the replies was made public here today. Belaunde said that both Colombia and Venezuela, where a number of Latin American foreign ministers met today to discuss the crisis, have offered their "most decided support" for the plan.