Despite an initial rejection by Argentina, diplomatic sources said yesterday there is still an outside chance that a Peruvian peace initiative launched with U.S. cooperation might evolve into a vehicle for renewed British-Argentine negotiations to resolve the Falkland Islands conflict.

The still-secret plan, whose existence was revealed Sunday by Peruvian President Fernando Belaunde Terry, seems to offer the best avenue presently available for averting ever-widening fighting and getting the two countries into negotiation. But it appeared to be stillborn when Argentina's military junta said Sunday that it could not accept the idea.

Yesterday, however, speculation arose that there still might be life in the initiative when Argentina announced that two of President Leopoldo Galtieri's highest-ranking military aides--Gen. Hector Iglesias and Rear Adm. Benito Moya--were being sent to the Peruvian capital of Lima for the apparent purpose of discussions with Belaunde.

U.S. officials said yesterday that they had insufficient information to judge whether the unexpected Argentine move offered any reason for hope that the initiative, put together by Belaunde in consultation with Secretary of State Alexander M. Haig Jr., might make some progress. But they also said the Reagan administration was watching developments and, as one source put it, "isn't quite ready to declare the initiative dead yet."

Well-informed diplomatic sources said the seven points in Belaunde's plan represented variations on an earlier U.S. proposal calling for a cease-fire, withdrawal of Argentine forces from the Falklands, an interim return to British administration under joint supervision of the United States, Britain and Argentina, and negotiations on the long-range status of the islands.

These sources said the initial Argentine rejection of Belaunde's ideas appeared to have been due to Argentina's continuing insistence that its demand for acknowledgment of its sovereignty over the islands is not negotiable--a demand unacceptable to Britain. However, the sources continued, the two emissaries from Buenos Aires were understood to be bringing to Lima some counterproposals that might enable Belaunde to keep his channel open.

Earlier yesterday, following a National Security Council meeting on the Falklands crisis, administration sources, citing Argentina's Sunday turndown of the initiative, said its chances of acceptance in Buenos Aires appeared literally to have been "torpedoed" by the apparent sinking of an Argentine cruiser, the General Belgrano, by a British submarine.

According to the sources, the sinking of the warship, with a possible heavy death toll among its crew, is certain to trigger an emotionally angry public reaction in Argentina that would work against any attempt to reach an accommodation with Britain at this point.

A similar situation last week helped derail efforts by Haig to act as mediator. After Britain recaptured South Georgia island from the Argentine forces that occupied it April 2, the Galtieri government reacted by refusing to discuss with Haig a U.S. peace plan, and the United States, finding itself unable to get the secretary's shuttle diplomacy back on track, publicly declared its support for Britain.

However, U.S. officials stressed that they would continue searching for a way to renew negotiations; it was this that led Washington to focus on the Belaunde initiative as a potentially acceptable vehicle.

Various diplomatic sources yesterday sketched this picture of its evolution. They said Belaunde decided he might be able to play a role because he is a democratically elected president who commands considerable respect in Latin America and because Peru, which has shown sympathy for Argentina's position in the Falklands dispute, enjoys good relations with Buenos Aires.

Initially, the sources continued, Belaunde planned to ask Canada, as a Western Hemisphere country openly sympathetic to Britain, to join with Peru in offering to mediate between the two sides.

However, before Canada could be approached, the Peruvian plan came to Washington's attention and resulted in telephone conversations between Haig and Belaunde that began Saturday and continued through Sunday until early yesterday. In these talks, a number of ideas were discussed that evolved into a proposal that the sources described as neither a purely American nor Peruvian plan, but an amalgam of views.

It was this proposal, the sources said, that Belaunde transmitted to Argentina on Sunday and whose status yesterday remained the subject of speculation after Argentina's confusing sequences of responses.