The United States and Peru yesterday were seeking to bring about a 72-hour cease-fire between Britain and Argentina as the first phase of a plan to resolve the South Atlantic crisis by withdrawing all military forces from the Falkland Islands area and beginning negotiations on its future.

Diplomatic sources said the cease-fire was the immediate objective of an intense, highly secret round of diplomacy being conducted by Secretary of State Alexander M. Haig Jr., working with Britain, and Peruvian President Fernando Belaunde Terry, dealing with Argentina.

However, while the sources said the U.S.-Peruvian initiative appeared to offer the best available hope of preventing further bloodshed, they stressed that, as of last night, it was still a long shot whose chances of successful implementation remained unclear.

In New York, U.N. Secretary General Javier Perez de Cuellar, who is Peruvian, said he had received a "positive reaction from the Argentine government" on a separate U.N. plan that bears similarities to the one being pursued by Haig and Belaunde.

U.N. officials said the Argentine response, which came in a message from Foreign Minister Nicanor Costa Mendez, was an acceptance of Perez de Cuellar's proposal only "in principle."

The officials said details of the proposal would become clear only after the secretary general meets with Costa Mendez.

Perez de Cuellar said he hoped to have a positive British response today, and added that he was "a little optimistic, not too much" about the prospects of his peace plan.

The State Department, in response to questions about the U.N. initiative, said yesterday that the United States would "welcome any forum or venue that would bring the conflict to a halt."

However, diplomatic sources stressed, the United States and Britain do not believe that the U.N. option offers much hope for quick and successful action, and they said the main focus of diplomatic activity is on the U.S.-Peruvian effort.

According to the sources, this initiative is the offshoot of a proposal put forward by Belaunde last weekend, only to be rejected by Argentina. However, the Argentines also are understood to have expressed willingness to discuss the idea further, and the sources said that Haig, who has been in close touch with the Peruvians since the weekend, subsequently proposed modifications that have become the main focus of efforts to end the Falklands crisis without further bloodshed.

The sources described the main elements of the plan as getting the cease-fire into effect as soon as possible and following it up with the removal of Argentine troops from the Falklands and the withdrawal of the British armada from the South Atlantic. That would restore the situation that prevailed before April 2, when Argentine forces occupied the islands.

After the dual withdrawal, the sources continued, there would be some form of internationally supervised, interim administration of the Falklands, and intensified, open-ended negotiations between Britain and Argentina to determine their ultimate status.

In its broad outline, the plan closely resembles a U.S. initiative that foundered because of Argentina's demand that any outcome acknowledge its sovereignty over the Falklands.

In fact, the sources said, the initial Argentine rejection of Belaunde's proposal was based on the continuing insistence of President Leopoldo Galtieri's military regime that the sovereignty issue is not negotiable.

However, the sources added, in the revised form under discussion the sovereignty question is couched in ambiguous language that reportedly can be interpreted as permitting Argentina to withdraw from the islands while reserving its position.

The sources said Belaunde, regarded in Buenos Aires as sympathetic to Argentina's position, has been working to convince the Galtieri government to show flexibility on this point, and the sources added that there were "some glimmers of hope" that he might be succeeding.

Another big problem, the sources continued, stems from British Prime Minister Margaret Thatcher's insistence that the wishes of the Falkland Island residents must be "paramount" in reaching any long-range solution. In a series of exchanges with London that intensified Tuesday night, Haig reportedly sought to induce the British to soften their stance on this issue.

According to the sources, Britain indicated that it would be willing to accept language stating that a solution take into account "the wishes and aspirations" of the islanders. However, that was described by the sources as still too strong for Argentina, which reportedly wants any cease-fire agreement to talk only about "the interests" of the islanders.

This was believed to have been a major discussion topic in meetings Tuesday and yesterday between Haig and British Ambassador Sir Nicholas Henderson.

So far the British have shown more interest in American mediation efforts than they have in any U.N. plan, saying that U.N. involvement could influence the outcome of talks on sovereignty in ways more favorable to Argentina.

But the British have found it difficult to reject Perez de Cuellar's proposals outright because of pressure at home and from allies in NATO and the Common Market.

Sources indicated that the chief elements of Perez de Cuellar's plan were cessation of hostilities, withdrawal of Argentine forces and the British fleet and appointment of a U.N. administrator while negotiations resume on the future of the islands. The plan reportedly takes no position on the islands' sovereignty.