Antigovernment guerrillas inflicted heavy damage on the Salvadoran Air Force early this morning when they set off a series of powerful bombs inside the Ilopango air base on the outskirts of this city.

At least five of the 14 American UH1H, or Huey, helicopters on loan to the Salvadorans and three of about 16 French-made jets were damaged by the blasts, according to sources close to the Salvadoran military. Some were destroyed; even aircraft inside hangars were badly damaged.

Civilians who saw parts of the field this morning indicated that the damage might be even more extensive, but the reports could not be confirmed.

More precise figures on the damage were impossible to obtain as the government barred access to the scene. Officials worked late into the day trying to determine the extent of the destruction and just how the attack was carried out.

At a hurriedly called press conference at midday, the defense minister, Gen. Jose Guillermo Garcia, declined to give specific information on the attack. He did confirm, however, that both helicopters and airplanes were damaged by the explosions, which were heard through much of the city as they went off like a string of enormous fireworks about 1:30 a.m.

Garcia said that the Salvadoran Air Force is still capable of operating as necessary and that several helicopters were able to take off during the assault to search, apparently without success, for the attackers. Garcia said that neither government forces, nor, as far as he knew, the guerrillas, suffered any casualties.

This afternoon three Hueys, four older observation helicopters and three jets crisscrossed the skies above this capital in an apparent attempt to show continued strength.

Sources with close ties to the Salvadoran Army said that the bombing this morning appears to have coordinated an attack from outside the base, possibly involving various types of projectiles, with extensive sabotage from within.

Garcia would characterize the incident only as a "terrorist act."

Garcia denied that any members of the armed forces were involved in sabotage, although the apparent lack of guerrilla casualties raises questions about how they managed to enter the heavily fenced and guarded field.

The bombing comes as the Salvadoran government is asking for increased military aid from the United States, including more of the Huey helicopters considered essential in fighting the insurgents. The American helicopters, mounted with heavy machine guns, are used for troop transport, medical evacuation and covering fire for soldiers on the ground. The jets can be used for strafing and light bombing missions.

On several occasions since the helicopters first arrived here a year ago as many as nine out of 10 have been inoperative because of ground fire and maintenance difficulties.

On Jan. 10 of last year a rebel attack on Ilopango aimed at crippling the jet squadron was a complete failure as several guerrilla rounds fired from recoilless rifles soared harmlessly over the garrison and the landing strip.

Today's bombing suggests that the insurgents' demolition skill and operational planning as well as their capacity to penetrate tight military security have improved dramatically.