Despite Nicaragua's acceptance of a U.S. call for negotiations, the Reagan administration is stalling the start of talks because of the Falklands crisis and a belief that the Nicaraguans are insincere and need to be pressured further to stop aiding El Salvador's leftist guerrillas.

Administration sources said yesterday that putting the negotiations on hold is, in part, unavoidable because Secretary of State Alexander M. Haig Jr. and Assistant Secretary Thomas O. Enders, who is expected to be the chief U.S. negotiator, are involved in the intercontinental shuttle effort aimed at averting warfare between Britain and Argentina over the Falklands.

But more important, the sources said, the administration does not believe that Nicaragua's revolutionary government is prepared, at this time, to negotiate in a way that shows any real promise of achieving the U.S. goal of cutting off the arms, guidance and other aid that the United States contends are flowing from Nicaragua to the Salvadoran insurgents.

Instead, the sources said, U.S. policy makers think that Washington's best strategy is to hold back in hopes that internal unrest directed at the leftist, Sandinista-dominated government and Nicaraguan fears about U.S. efforts to encourage this dissidence will, as one source put it, "soften the Sandinistas up and make them more inclined to negotiate seriously on the terms we've proposed."

Specifically, the sources said, the administration does not believe the Nicaraguans were sincere in their offer, conveyed to the United States on Wednesday, "to begin serious and formal negotiations," including discussion of an eight-point proposal put forward by Washington last week.

To support that contention, the sources said the United States has found no sign that Nicaragua is acting to halt or reduce the arms flow into El Salvador. They added that, until there is verifiable evidence of Nicaragua clamping down on the arms traffic, Washington does not intend to move beyond its current stance of saying it is "studying" Nicaragua's offer to negotiate.

"We don't think it's a serious offer, and we don't intend to give it any priority, certainly not while the Falklands crisis is alive and, given our evaluation of their intentions, not any time soon after the Falklands matter is resolved," one source said.

At a time when world attention has been diverted away from Central America to the possibility of a British-Argentine clash in the South Atlantic, the administration's delaying tactics toward Nicaragua are unlikely to attract much notice. But, if a settlement of the Falklands situation is achieved, the administration could find itself enmeshed in a new controversy about whether its offer to negotiate with Nicaragua was made in good faith.

The administration never has made any secret of its belief that talks with Nicaragua are likely to prove unsuccessful. Until now, however, the administration has taken the position that it is committed to go ahead with such talks, partly as a gesture toward Mexican President Jose Lopez Portillo, who has acted as an intermediary between the two countries, and partly to guard itself against the charge of congressional critics that it is interested only in a military solution to the Salvadoran civil war.

This charge is almost certain to be heard again if the administration is perceived as stonewalling in the face of Nicaragua's publicly stated willingness to begin immediate talks on any subject that Washington wants to discuss including the U.S.-originated, eight-point plan.

Nicaragua, in effect, would be in a position to reap a potential propaganda bonanza in Latin America and Western Europe by charging that the United States, after calling for negotiations and receiving a positive response, now is seeking to set new conditions for going ahead.

A prolonged U.S. reluctance to start talks could also exacerbate strains in Washington's relations with the Mexican government, which has staked much of its international prestige on its efforts to act as a peacemaker in Central America.

Despite these risks, the sources said the administration believes that time is on its side in dealing with Nicaragua because of the Sandinista regime's need to guard itself against internal dissatisfaction with the revolution and its fear that the United States will aid dissidents like Eden Pastora, a disaffected revolutionary leader, in campaigns to displace the country's leaders.

The regime has placed the country under an official state of seige because of its nervousness over alleged U.S. covert action programs directed against it, and yesterday it accused the United States of sending two destroyers to cruise close to its Caribbean coast as part of Washington's "interventionist military preparations."