USULUTAN, EL SALVADOR, FEB. 17 -- Leftist rebels, in their biggest action in nine months, attacked a central provincial capital today, killing eight soldiers and seven civilians and wounding more than a dozen residents, according to civilian and military sources.

Witnesses of the attack on this center of the cotton-growing region said the civilians were killed when a rebel mortar shell -- aimed at the headquarters of the Army's 6th Brigade -- fell on a nearby house, blowing out the walls and killing an entire family.

One of the four children killed was 6 months old. Two U.S. advisers were at the brigade headquarters during the attack, a Salvadoran officer said.

The brigade, apparently the main object of the attack, is located in the center of the city, surrounded by houses. Several other rebel explosives fell about 20 yards short of the brigade walls, seriously damaging at least six other houses and wounding several civilians.

"It began right after midnight, and with the first blast, our door was blown off. We threw ourselves on the floor and began to pray," said Rosa Santos, who lived in a house that was badly damaged. "We are only alive by the grace of God."

Fighting lasted about 2 1/2 hours, civilians said. Scores of people picked through the rubble of their destroyed homes, salvaging belongings or staring at the damage. Usulutan is 55 miles southeast of the capital, San Salvador.

Rebels of the Marxist-led Farabundo Marti National Liberation Front (FMLN) simultaneously blew up two bridges, one on the vital Pan-American Highway and one on the coastal highway, cutting off traffic temporarily.

The rebels also knocked down a score of electric pylons, burned six warehouses full of cotton on the outskirts of town and destroyed a vegetable-oil factory. No immediate estimate of the damage was available.

The rebels failed to penetrate the military compound and only succeeded in damaging the kitchen and eating area. Most of the heavy fighting took place around a training facility outside the garrison.

This was the largest coordinated FMLN attack since May 1987, when the insurgents tried to take the military headquarters in San Francisco Gotera in Morazan province.

The attack came as five Central American foreign ministers met in San Salvador to debate ways of verifying that each nation was complying with a regional peace plan.

Salvadoran Foreign Minister Ricardo Acevedo Peralta said one of El Salvador's top priorities was to denounce what he said was continuing support for the FMLN by Nicaragua's leftist Sandinista government.

Lt. Armando Brenes, who helped lead the defense against the attack here, said casualties would have been much higher if an Army patrol had not run into some rebels about a mile outside the city 30 minutes before the combat began, giving the troops time to prepare.

Brenes said the Air Force sent helicopters from the capital within an hour, helping to drive back the rebel force, estimated at between 150 and 200 men. The insurgents scattered leaflets urging people not to participate in the March 20 legislative and municipal elections, and promising more attacks soon.

Col. Mauricio Ernesto Vargas, chief of military operations, who was visiting the site, said the two U.S. military advisers who were at the 6th Brigade garrison during the attack were never in danger and were not involved in the fighting.

El Salvador is one of the staunchest U.S. allies in the region, receiving $85 million in military aid this year. The United States maintains up to 55 military advisers in El Salvador to help direct the war, which has claimed some 61,000 lives in eight years.

In a special broadcast over clandestine Radio Venceremos, the rebels claimed to have inflicted 190 casualties on the military in the attack. There was no evidence of such losses. The rebels also vowed to launch soon their first nationwide transportation stoppage of the year.

Gen. Adolfo Blandon, armed forces chief of staff, said that five soldiers were killed in the attack on the brigade and 13 were wounded, and that two rebels also died there. Two soldiers were killed in a simultaneous attack on the nearby barracks of the Atonal Battalion, and one soldier guarding the cotton warehouses was killed.

A funeral home held eight coffins of the dead soldiers, and the morgue had the bodies of seven civilians. The bodies of two rebels were laid out on the training ground.

This was the first attack on Usulutan since 1982. The cotton industry is a major earner of foreign exchange.