Clearly referring to Nicaraguan-backed guerrillas in El Salvador, President Daniel Ortega declared today that "any irregular force in the area" could get and use antiaircraft missiles now that U.S.-supported rebels have used them to shoot down a Nicaraguan helicopter.

The warning appeared designed to dramatize official outrage at the rebels' successful use of such missiles last Monday for the first time in their four-year-old war to overthrow the Sandinista leaders of Nicaragua.

In Washington, the State Department made public what Secretary George P. Shultz called "incontrovertible evidence" that large numbers of Cuban soldiers are heavily involved in combat against the rebels, known as contras or counterrevolutionaries, Washington Post staff writer Joanne Omang reported.

Made up primarily of selected declassified portions of testimony from Sandinista defectors, the 14-page document said Cuban advisers wear Sandinista uniforms, pilot Soviet-built helicopters and sometimes engage in combat.

Ortega, speaking at a news conference, repeated Nicaraguan charges that the Reagan administration directly supplied the Honduras-based rebel forces with SA7 ground-to-air missiles, saying this represented a "terrorist escalation" of Central American conflicts.

"Here we have the U.S. government, which calls itself a fighter against international terrorism, itself stimulating international terrorism," he said, adding later: "They are opening the door for any irregular force in Latin America to use this type of arm."

In Washington, Shultz denied that the United States had supplied the contras with SA7s but added that he was delighted that they have the missiles.

"Thank goodness that they did get hold of some weapons that can knock these choppers down," he said. "If I were them I would certainly want to" have such arms, he added later. "They have gotten ahold of missiles and figured out how to use them, . . . and I say, 'Fine, I'm all for it.' "

A highly placed diplomatic source said the contras obtained the SA7s from a private arms dealer in Portugal about six months ago.

The Salvadoran guerrilla alliance, the Farabundo Marti National Liberation Front, has encouraged discussion for the past several years about whether its combatants could obtain antiaircraft missiles. The Nicaraguan military has a substantial number of the missiles, mostly Soviet-made SA7s.

Such weapons would mark a substantial shift in the Salvadoran civil war, where the U.S.-supplied Air Force increasingly has been relying on aircraft for bombing, strafing and rocketing and for transporting troops for swift assaults. But the portable heat-seeking missiles are not known to have been supplied to the Salvadoran guerrillas.

The main U.S.-sponsored Nicaraguan rebel group, the Nicaraguan Democratic Force, announced early this year that it had acquired a number of the shoulder-fired missiles and was training its men in their use at camps in the Honduran border hills. But Monday's downing of a Nicaraguan Air Force Mi8 helicopter marked the first known occasion that the guerrillas brought down one of Nicaragua's more than 18 Soviet-made military helicopters with the missiles.

At a press conference in Washington, Shultz called Nicaragua "a cancer in the region" and added that "further steps" against it are possible. He said the Soviet Union has "been putting military things in there as fast as they can, as fast as they think it can be absorbed." He added, "I suppose you can say that amounts to a military base."

Shultz hinted that he might ask soon for renewed military aid to the contras. Congress cut off covert arms supplies to the rebels in 1984 but approved $27 million in nonlethal aid in August. That aid runs out in March.

Ortega refused three times to say whether Cuban military advisers were killed when the aircraft went down near an Army training base at Mulukuku in the central Nicaraguan mountains. The U.S. assistant secretary of state for inter-American affairs, Elliott Abrams, said in Washington yesterday that Cubans perished in the crash, citing intelligence reports and sightings by rebel forces.

The Nicaraguan Defense Ministry reported yesterday that 14 "members of the Popular Sandinista Army" were killed, without specifying their nationality. The newspaper Barricada, the official organ of the Sandinista National Liberation Front, listed the casualties by name and provided Nicaraguan home towns for all 14.

Ortega said Abrams complained of Cuban advisers in Nicaragua "to cover the terrorist action of providing missiles to the contras."

Ortega also declined to respond directly to Abrams' charge that Cuban advisers increasingly have begun to participate in combat alongside Nicaraguan soldiers. But he insisted that the United States regularly exaggerates the number of Cuban military advisers stationed here.

Nicaraguan officials have said about 2,000 Cuban advisers in all fields are in Nicaragua, including about 700 military advisers mostly engaged in training Nicaraguan soldiers. Abrams put the number of military advisers alone at 2,500.

A spokesman for the Cuban mission in Washington, Angel Pino, said the administration's charges were an effort "to justify the contra escalation," their first use of SA7 rockets. He said there had been no recent changes in Cuban-American relations and no change in the level of Cuban involvement in Nicaragua.

"This is all directed toward the internal audience here," Pino said. "It is propaganda to convince Congress to give more aid" to the contras.

Ortega dismissed as "cynical" a message that he said was given this afternoon by the State Department to the Nicaraguan charge d'affaires in Washington, Manuel Cordero. The Nicaraguan ambassador to Washington, Carlos Tunnermann, was recalled for consultations yesterday as part of Nicaragua's protest against use of the missiles.

As described by Ortega, the U.S. note said that the Nicaraguan Democratic Force, not the United States, shot down the helicopter, adding that introduction of the missiles is defensive, because Nicaragua's use of Soviet-made helicopters has escalated the conflict.

"This note does not at all deny that the United States, through the Central Intelligence Agency, provided the missiles to the contras," Ortega said. "This note is cynical, totally cynical. I cannot give any other explanation of this note."