SAN SALVADOR, NOV. 23 -- Leftist rebels shot down a Salvadoran air force jet today, the government said, apparently the first time the guerrillas have successfully used an anti-aircraft missile in El Salvador's long civil war.

The A-37 attack jet's pilot was killed when the aircraft was downed by a missile in Uslutan province in the eastern section of the Central American country, the government said in a statement broadcast on national radio. The Farabundo Marti National Liberation Front (FMLN) rebels did not issue a statement on the aircraft's downing.

Although the rebels have acknowledged possessing Soviet-designed SA-7 missiles, popularly known as SAM-7s, they have never previously used them to down government aircraft, military analysts said.

The armed forces press office initially said the plane crashed because of mechanical failure.

The rebels launched a nationwide series of military attacks Monday night, which have claimed at least 90 lives and wounded some 300 people.

"The FMLN {rebels}, during the last 72 hours, have been using sophisticated war devices, such as SAM-7 and SAM-14 ground-to-air missiles," the government said in a statement broadcast on national radio. "These constitute extremely dangerous actions which could affect the country's political stability," the statement said.

Some analysts said that if the rebels began to use such missiles they could tip the military balance in their favor in the Central American country's 11-year civil war by neutralizing the government's air power.

"This is a serious blow to the peace process. . . . It shows the FMLN are laughing at their commitment with the secretary-general of the United Nations to seek with good faith a peaceful solution to the armed conflict," the government said.

U.N.-sponsored peace talks to end the war in El Salvador are deadlocked. Some 75,000 people have died in the conflict.

Last November, during a rebel offensive, a light plane carrying SAM-7 missiles and one American-designed Redeye missile crashed in central El Salvador. The military said the missiles were destined for the rebels and that another planeload got through to them.