President Reagan and Secretary of State George P. Shultz yesterday told Republican congressional leaders that the military situation has improved in El Salvador and with it the prospects for a diplomatic settlement of the conflict in Central America.

"Things are moving in a reasonably positive direction," Shultz was quoted by White House spokesman Larry Speakes as telling the congressmen.

Administration officials said Speakes was sent out to brief reporters on the hour-long meeting after the GOP congressmen gave only a mildly optimistic report of a session that was designed to demonstrate that Reagan's policy is succeeding in Central America.

"There is no new development that we see in prospect," Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) told reporters on the White House driveway after the meeting. "It appears marginally that things are improving."

Soon afterward, Speakes appeared in the White House briefing room with a rosier assessment. He cited a number of positive developments, including the military situation in El Salvador, the efforts of other countries led by the Contadora group of Colombia, Mexico, Panama and Venezuela to find a peaceful settlement and a more conciliatory attitude by Cuba and Nicaragua.

"There has been a shift of position, at least rhetorically, on the part of Cuba and Nicaragua," Speakes quoted Shultz as saying.

United Press International reported last night that an unclassified Defense Department memo distributed to senior House Republicans said that as many as 5,000 U.S. troops may take part in military exercises in Honduras later this year. The Pentagon had previously estimated that no more than 4,000 U.S. troops would take part.

Cuban President Fidel Castro last week proposed a pact in which all parties in Central America agree to halt the supply of weapons and military advisers to El Salvador and Nicaragua. Baker yesterday called this proposal "worthwhile," and Reagan has said he will give Castro "the benefit of the doubt in any negotiations."

In addition to the Castro overture, U.S. special envoy Richard B. Stone on Monday concluded a nine-nation swing of the region by meeting in Managua, Nicaragua, with leaders of the leftist Sandinista government. Afterward, both Stone and Nicaraguan Foreign Minister Miguel d'Escota described the talks as "useful."

On Sunday, Stone met for two hours in Bogota with Ruben Zamora, a director of the political arm of the five leftist guerrilla groups that are striving to bring down the government in El Salvador.

Rep. Silvio O. Conte (R-Mass.), one of the congressmen briefed at the White House, said that Shultz "talked in favorable terms about the Stone mission."

Stone talked separately yesterday with the secretary of state and national security affairs adviser William P. Clark to brief them on his trip. He is scheduled to meet today with the president.

While the White House has declined to give a formal assessment of the Stone negotiations, sources said the special envoy is likely to engage in further talks in an effort to promote a diplomatic settlement.

Stone has declined to talk publicly about the negotiations, saying that confidentiality is essential to the diplomatic effort he is pursuing. In addition, administration officials have made it clear that they do not want Stone talking to reporters.

Last week, a news account said Stone was overheard telling Salvadoran legislators that U.S. ships maneuvering off the Nicaraguan coast could be used for "attack" purposes. The State Department said this account was "totally inaccurate."

Yesterday, Speakes said that Shultz told the congressmen that the 19-ship U.S. naval force, which will be on maneuvers for six months, has a first purpose of training but will also "show to friends and adversaries alike that the United States has the will and ability to mobilize sufficient force in an area quickly."

"If that has a deterrent effect, so be it," Shultz was quoted as saying.

Rep. Richard B. Cheney (R-Wyo.), who attended the White House briefing, was even blunter.

"We've had exercises down there for years," Cheney told reporters. "Obviously, this is somewhat different. It's a show of force."

On the Senate floor yesterday, Senate Minority Leader Robert C. Byrd (D-W.Va.) urged the administration to scale back the maneuvers to test the sincerity of the Cubans and the Nicaraguans.