The Reagan administration today authorized Mexico to propose that Cuba and Nicaragua stop aiding El Salvador's leftist guerrillas in exchange for guaranteeing Nicaragua against intervention by the United States or hostile Central American countries.

Following a meeting here with Secretary of State Alexander M. Haig Jr., Mexican Foreign Minister Jorge Castaneda described the U.S. "ideas" as envisioning a series of nonaggression pacts between Nicaragua and its neighbors. That, he added, would be coupled with U.S. promises not to train or assist insurgent forces that seek to overthrow Nicaragua's revolutionary government.

He said the U.S. proposals would be conveyed to Cuba and Nicaragua for reaction within the next few days and that he was considering visiting these countries to discuss the situation with their leaders.

Today's session marked the latest follow-up to the peace proposals advanced by Mexican President Jose Lopez Portillo in Managua, Nicaragua, three weeks ago. The proposals have started to assume the dimensions of a potentially important factor in efforts to resolve the tensions generated in Central America by El Salvador's civil war.

In a meeting here with Castaneda on March 7, Haig stressed that "an essential and primary element" of any solution, from the U.S. point of view, requires that Nicaragua halt its alleged supply of arms and other aid to the guerrillas fighting the U.S.-backed civilian-military government in El Salvador.

Today Haig gave Castaneda an outline of what the United States and its allies in Central America might be willing to do in exchange for an end to Nicaraguan and Cuban involvement in the Salvadoran conflict. Stressing that the United States appeared willing to make concessions, the Mexican minister said:

"Secretary Haig did implicitly agree that the commitment by Nicaragua would have to be accompanied by other measures."

Asked what form the U.S. concessions might take, both Haig and Castaneda dodged discussion of alleged U.S. "covert action" directed against the Sandinista-controlled government in Nicaragua. In fact, both said the subject was not discussed.

Still, Castaneda left no doubt that the proposals deal with Nicaraguan fears, swelled by leaks in Washington in recent days, that the United States is preparing a covert action program.

In response to questions about covert action, Castaneda said: "We didn't speak specifically about that. But, of course, if all parties agree to a set of quid pro quos, naturally one of them would have to be that all these activities are stopped."

At another point, he said the United States would have to promise "not to train or not to allow the training within the United States" of followers of the late Nicaraguan dictator Anastasio Somoza, and "to take more severe measures against these activities." Somoza was ousted by the Sandinistas in 1979, and the current Nicaraguan government charges that Washington is helping to train exiles from Somoza's old army operating in the United States and Honduras.

In his public comments about today's meeting, Haig stuck to generalities and refused to predict whether Mexico's initiative might lead to a resolution of the Central American crisis.

But Castaneda, speaking separately with reporters, seemed optimistic that the United States, which initially reacted very coolly to Lopez Portillo's mediation proposal, now views the idea as a potentially important vehicle for dealing with two of what the Mexican president called the three "knots" of tension in the region--specifically, U.S.-Nicaraguan and U.S.-Cuban relations.

In regard to Nicaragua, Castaneda said that a solution must involve "something that is worthwhile and valuable for the Nicaraguans. For Nicaragua, that would be to feel secure from foreign dangers. They feel threatened by a higher level of arms in the hands of their neighbors. They feel threatened by the Somoza people."

He said these fears could partly be set to rest through nonaggression pacts. Although he did not specify, that probably would involve El Salvador, Honduras and Guatemala, all with strong rightist military influence hostile to the revolutionary government of Nicaragua.

As to the U.S. role in any deal, Castaneda noted that Haig "has not dropped" the U.S. insistence that Nicaragua end its involvement in El Salvador. But, in an apparent reference to the proposed exchange of concessions, he added:

"I feel we now have a clearer picture of the context in which that operates.

"It must happen at the same time that other things happen."

He also appeared optimistic about a possible easing of tensions between Washington and Cuba. The Mexican minister said Cuban President Fidel Castro has assured Lopez Portillo that he is serious about negotiating. Castaneda added:

"We are convinced" that the Cubans "are eager to negotiate what could amount, in the end, to normalization of relations with the United States."

As to the so-called "third knot of tension" in the region--the Salvadoran civil war--Castaneda conceded that there still is a substantial gap between Mexico's call for negotiations with the guerrillas and Washington's lack of enthusiasm for that idea. The minister said he and Haig will meet again to discuss the Salvadoran situation after the March 28 elections there for a constituent assembly.

He also disagreed with Haig on whether tensions in Central America involve essentially regional issues or are part of a global problem.

On Saturday, an anonymous senior State Department official, describing Haig's thinking, said the secretary saw the Central America situation in a global context involving the intervention of the Soviet Union as well as Cuba and Nicaragua.

At a news conference today, Haig elaborated by saying that much of the American press and public has "a myopic preoccupation with El Salvador as a self-sustaining problem where all the ingredients are internal. It's not that at all, no more than Vietnam was."

He charged that "Russian resources and Russian armaments already are engaged in this hemisphere," and added: "They are involved. They clearly have a responsibility in bringing these events to a peaceful outcome."

But he stressed that the United States plans to pursue the question of Soviet involvement through bilateral talks with Moscow rather than attempting to bring the Soviets into some kind of international negotiations over Central America. "It does not mean there is a direct role for the Soviet Union in this hemisphere," he said.

Haig and Castaneda met prior to a larger session today and Monday in which they will join the foreign ministers of Canada, Venezuela and Colombia to discuss ways of implementing President Reagan's Caribbean Basin initiative by more closely coordinating their aid to the region.