The Reagan administration, racing against time to avoid a cutoff of U.S. aid to Nicaragua, is putting heavy pressure on that Central American country's revolutionary government to halt what U.S. officials contend is a heavy flow of arms through its territory to leftist guerrillas in neighboring El Salvador.

Reliable sources said yesterday the effort has so far succeeded in causing Nicaraguan authorities to close down a clandestine radio station broadcasting revolutionary propaganda into El Salvador.

But, the sources continued, Nicaragua's leftist-dominated government, which officially denies aiding the Salvadoran guerrillas, has told Washington it needs more time -- perhaps two weeks or more -- to investigate properly U.S. evidence about the alleged arms flow and take steps to deal with it.

The sources revealed that Lawrence Pezzullo, the U.S. ambassador in Managua, was called to Washington without any announcement Wednesday night for consultations on whether the Nicaraguans appear serious about cooperating in choking off the arms flow and, if so, how much time they will need to start showing results.

The administration's problem, the sources said, is that it may not have that much time because conservative forces in Congress, led by Sen. Jesse Helms (R-N.C.), are pressing the White House and State Department for an immediate determination of whether Nicaragua is violating the terms of the $75 million aid package put together by the Carter administration to help Nicaragua with its massive economic problems.

In voting the aid, Congress stipulated that the president certify that Nicaragua is not assisting guerrilla or terrorist movements in other Central American countries. Despite this, U.S. officials privately have made clear, Washington has accumulated substantial evidence that arms, much of them from Cuba, are being shipped to the Salvadoran guerrillas through Nicaragua with what former secretary of state Edmund S. Muskie has called "the knowledge and to some extent the help" of Nicaraguan authorities.

A major Reagan administration goal is to help the civilian-military coalition ruling El Salvador thwart the leftists trying to win power in the civil war there. But, the sources said, for the moment at least, the new administration has been heeding the counsel of the State Department's Latin American experts and trying to avoid a break that could put Nicaragua firmly under the influence of Cuban President Fidel Castro and his communist system.

At present, the undisbursed balance of the $75 million, which amounts to $15 million, and Food for Peace aid totaling another $15 million are being withheld from Nicaragua while the administration determines whether the conditions set by Congress are being met. In private, U.S. officials say candidly that if the certification had to be made within the next few days, it would require cutting off the aid.

But, while Reagan in his election campaign attacked the Sandinista Liberation Front dominating the Nicaraguan government as "Marxist," his admistration so far appears to be heeding State Department warnings that an abrupt termination of the aid could cause economic chaos and renewed violence within Nicaragua, undercut moderate elements there struggling to keep the country out of Cuba's orbit, and eliminate the last vestiges of U.S. leverage to keep Nicaragua out of the neighboring El Salvador conflict.

The emphasis on El Salvador was underscored anew yesterday by State Department spokesman William Dyess, who said the primary threat to its government is seen coming from the leftist guerrillas rather than rightist extremists and U.S. military aid may be increased if needed.