Rebels massacred 13 coca harvesters and their cook, the latest victims of a feud among paramilitary gangs fighting to control Colombia's lucrative cocaine trade, authorities said Thursday.

Members of the Revolutionary Armed Forces of Colombia, or FARC, gunned down Flor Maria Gutierrez and the 13 men as she served them lunch Wednesday on a farm near the village of Puerto Valdivia, 220 miles northwest of the capital, Bogota.

The victims worked on one of the many farms in Antioquia state, where coca flourishes. The men stripped coca bushes of their leaves, ground them and then added chemicals to make coca paste, a key step in producing cocaine.

They had elicited the rebels' wrath by selling their product to a right-wing paramilitary faction, said Antioquia's deputy governor, Jorge Mejia.

Each of the warring factions aims to terrify the harvesters and drug lab workers into selling their product exclusively to it. The illegal groups then have the coca paste processed into purified cocaine and export the drug abroad. The farmers who grow coca are often caught in the middle.

According to news reports, rebels of the FARC's 36th Front barged onto the farm, threatened the workers and opened fire. Colombian army counterinsurgency troops were deployed to the region and clashed with the rebels, killing three, the chief of the Colombian National Police said.

A woman mourns a coca harvester killed by rebels in northwestern Colombia.