The White House is expected to decide soon whether to send an additional $25 million-$30 million in military supplies to El Salvador -- including helicopters, patrol boats, ammunition and spare parts -- and as many as 26 more U.S. military personnel who would help train Salvadoran forces but would not be involved in combat.

From President Reagan on down, administration officials yesterday stressed repeatedly that the United States has no plans to send combat troops into the tiny and troubled Central American republic nor is there any prospect of a Vietnam-like involvement there.

"I know there is great concern over that," Reagan said in a brief meeting with reporters yesterday. "I think it's part of a Vietnam syndrome. But we have no intention of that kind of involvement.

"There is no question," however, the president added, "that we are in support of the government there and against those who believe in the violent overthrow" of that government.

Administration sources privately said the proposals on more military equipment and about two dozen additional military training personnel, mostly Army and Navy, were at the moment the most extensive options likely to be set before the president as the United States tries to beef up the Salvadoran junta's ability to cope with leftist guerrilla forces.

The sources said there was no move now being planned that would involve major U.S. naval action either against Cuba or along the El Salvador coast to stop any seaborne flow of arms from Cuba or Nicaragua, though some newspaper reports yesterday may have conveyed that impression.

"There is more action in the newspapers than in the Pentagon," one senior naval officer said, explaining that the Defense Department has not been ordered to do anything involving the simmering situation in Central America.

Several officials said that while nothing has been ruled out and that the United States might eventually have to take stronger military measures aimed at Cuba, the general feeling is that the administration is still a long way from such action.

Aside from the prospect of more aid and advisers, officials said the United States would also soon strengthen its intelligence-gathering operations in, around and over Nicaragua to try to determine with assurance if the clandestine arms flow into El Salvador has, in fact, been slowed or shut down by the Nicaraguans.

The United States has been delaying about $30 million in financial aid to Nicaragua, demanding that the leftist government there put a stop to the illegal arms that have been moving through Nicaragua from Cuba and into El Salvador. The State Department, officials said, "wants to be damn sure" that the arms flow is actually stopped before the president is given any recommendation to resume economic aid to Nicaragua.

At the State Department yesterday, spokesman William Dyess said that the arms flow "appears to have slowed down in the last couple of weeks" but that the United States has no corroboration that it has stopped. Once before, Dyess said, the flow stopped but started again.

Nicaraguan officials reportedly have told the administration here they will try to stop the arms traffic, but Dyess made clear yesterday that the United States "is looking for deeds, not words. We want a complete shutdown of the arms flow."

As of yesterday, Dyess said, there had been no decision on whether to send U.S. naval advisers to El Salvador, though officials have said previously that helping the Salvadorans set up a patrol off their Pacific coast to intercept arms coming by sea from Nicaragua would probably be a good and relatively safe tactic.

The U.S. Navy happens to have a fleet of about 41 warships, including an aircraft carrier, involved in a training exercise in and around the Caribbean.

These vessels and aircraft clearly could be used for some kind of blockade of Cuba, if that were planned, or to enforce some kind of U.S. threat to take action against vessels that cross a certain line off the El Salvador coast.

Both civilian and Navy officials said yesteday, however, that the U.S. fleet exercise is an annual, six-week event that began Feb. 3, was planned long ago, was even larger last year and had nothing to do with the administration's current problems with gun running to El Salvador.

U.S. fleet activity in the region has been stepped up since the discovery of the Soviet brigade in Cuba in the fall of 1979.

The bulk of arms entering Nicaragua comes by air from Cuba and then is smuggled into El Salvador by land, sea and air via Nicaragua and Honduras, according to the State Department. But El Salvador's only coast is on the Pacific, with no U.S. Navy warships currently nearby.