Striking banana workers are returning to their jobs following violence that paralyzed production in Colombia's most important banana zone.

The banana workers' union said 600 banana workers have been killed so far this year in the Uraba region. Conflicts raging among Marxist guerrillas, ex- guerrillas and paramilitary groups fighting for territory and political control have resulted in several massacres and caused 12,000 residents to leave this poor Caribbean region.After 17 banana workers en route to work last week were forced off their bus, marched to a soccer field and executed, union leaders said "Enough!" Uraba's 11,000 banana workers called a strike Aug. 30, refusing to go back to work until the government promised to assure their safety.

"Everyone was terrified, too frightened to go to the plantations," said union spokesman Ramon Osorio.

Workers began returning this week in the wake of promises by Colombian President Ernesto Samper to beef up security and social services. The number of soldiers patrolling the plantations has been increased to 500 from 100.

But, according to Mr. Osorio, things haven't changed. "It's impossible to talk about improved security when the paramilitary groups continue killing people," he said.

Economic necessity has driven the laborers back to work, said Mr. Osorio. The strike cost the workers, who make an average $10.50 a day, $630,000 in lost wages.

In total, the six-day strike produced losses of $4.45 million, said Sabina Alvaraz of the Banana Companies Association. The 400 banana plantations in the region normally produce about 200,000 40-pound boxes of bananas for export each day.

Authorities believe the killers in the latest massacre, the third such slaughter in a month, belong to one of the country's main guerrilla groups, the Revolutionary Armed Forces of Colombia. The lawlessness has gone on for years and has transformed Uraba, nestled in the northwest corner of the country near Panama, into Colombia's most violent region.

The turmoil has curbed production in Uraba, which produces about 60 percent of Colombia's bananas, the association said in a recent newsletter.

"The intimidation, threats, extortion and terrorism have impeded management," the association said. "The banana plantations are operating at (low) productivity levels."

Banana companies are unable to inspect and manage the plantations properly, resulting in inadequate fertilization, pest control and harvesting.

Following a period of steady increases, the area under banana cultivation in Uraba has fallen in recent years to 68,048 acres from 71,220 acres, said Mr. Alvaraz. In the first six months of this year, Uraba exported 5 million fewer boxes than in the same period last year, representing a 20 percent decrease.

The country's drug traffickers extract their own toll on banana growers. Companies must pay a "tax" to guarantee safe shipment of their cargo and ensure that drugs are not hidden among the fruit to be smuggled to destination ports, said the association.

Colombia is the third-largest banana producer in the world, after Ecuador and Costa Rica. Bananas are the country's largest nontraditional export, worth about $450 million last year.

Colombia shipped 34 million boxes of bananas worth $185.7 million in the first six months of this year, down 4.7 percent in dollar terms from 40.3 million boxes worth $194.9 million for the same period in 1994.