MIAMI, JAN. 2 -- Nicaragua's Sandinista-controlled army has announced the arrest of four officers on charges of having sold 28 antiaircraft missiles to El Salvador's leftist guerrillas.

At least two of the weapons, which included 16 sophisticated, Soviet-made SA-14 surface-to-air missiles, were used by the Salvadoran guerrillas to shoot down two Salvadoran air force planes, in November and December. The missiles offer an important advantage for the guerrillas in the 11-year-old civil war.

In a communique issued Tuesday, the Sandinista Popular Army, which has maintained virtual autonomy despite the leftist party's electoral defeat last year by conservative President Violeta Chamorro, denounced the sale of the missiles as a "grave crime." It said the sale occurred in October.

The communique added that the Soviet Union, which supplied the weapons to Nicaragua in 1986, had named a commission to investigate the sale, which appears to violate agreements between Moscow and Managua banning the transfer of arms to third parties.

After the Salvadoran air force planes were shot down, U.S. and Salvadoran officials recovered the launch tube of an SA-14 rocket used by the guerrillas and supplied the serial number to the Soviet Union. The Soviets confirmed that the missile was part of a shipment to Nicaragua in February 1986, at the height of the Sandinistas' war against the U.S.-backed contra rebels.

The United States has maintained for more than a decade that the Sandinistas were shipping arms to the guerrillas of the leftist Farabundo Marti National Liberation Front (FMLN) in El Salvador.

The Sandinistas had never admitted it, even when confronted with clear evidence, such as the crash of a small plane laden with antiaircraft missiles in eastern El Salvador in November 1989. As late as Monday, a Sandinista army spokesman told the Associated Press that the latest reports were part of a propaganda campaign designed by the United States to discredit the Sandinistas.

But in a sudden reversal, the Sandinista army general command issued a three-page communique Tuesday asserting that "preliminary investigations" showed that the FMLN had purchased the missiles in "clandestine meetings in Managua" in October.

The communique said the FMLN, represented by Joaquin Villalobos, a top-ranking guerrilla commander, and another man identified only as "Rodrigo," had paid "thousands of dollars" for the missiles. The statement said the weapons had been stolen and sold without authorization of the Sandinista army.

According to the release, those arrested included a former major, three captains and 11 Salvadoran nationals. One of the officers was identified as Capt. Reynerio Padilla Alvarez, chief of the antiaircraft artillery regiment of the Sandinista air force.

U.S. officials contacted today were wary of the Sandinistas' announcement, unsure whether it reflected pressure from Moscow to stem the flow of arms traffic in the region or some other motive.

"I think they're really feeling the heat {from Moscow} on this," said one diplomat. But he added that the arrests might only signal an internal intrigue in which the army chief, Gen. Humberto Ortega, could be using the arms sales as a pretext to purge the military of his adversaries.

Ortega, the brother of ex-president Daniel Ortega, fired the air force chief of staff last year in an incident that left diplomats speculating about rifts in the armed forces' command structure.

A senior State Department official said, "The facts will speak for themselves. . . . If the arms shipments stop and the command and control and radio operations {of the FMLN} in Nicaragua stop, then we'll take seriously that Humberto Ortega intends to comply with Nicaraguan law and the Central American peace treaty that prohibits this kind of aid to violent revolutionaries."