Authorities did not yet have a motive for the attack on Tuesday near La Gabarra, a local mayor, Taiz Ortega, told The Associated Press by telephone. The town is 310 miles northeast of the capital, Bogota.
The attack appeared to be the work of the Revolutionary Armed Forces of Colombia, or FARC, Colombia's largest guerrilla group, which has been fighting to topple the government for 40 years.
Ortega said the victims apparently worked in fields of coca — the raw ingredient in cocaine — but that the ranch also produced cocoa, the main ingredient for chocolate, and other agricultural products.
The workers were sleeping in hammocks at the ranch when gunmen burst through the doors at dawn, tied the workers up with the hammocks' ropes and shot them with automatic weapons, Ortega said.
Yinith Guerrero, a regional human rights activist, said from La Gabarra that villagers were fleeing the area as news of the massacre spread.
At least five people were injured and taken by boat to a hospital in the nearby town of Cucuta.
"We saved ourselves by running toward the mountain," Jesus Bayona, 45, who was shot in the foot, told AP from his hospital bed.
The La Duquesa ranch is nestled amid the thick jungle and steep mountains of the Norte de Santander province, where FARC rebels and their right-wing paramilitary foes are locked in a bitter struggle for control of the lucrative drugs trade.
The top commander of the main paramilitary umbrella group known as the United Self-Defense Forces of Colombia, or AUC, denied involvement in the killing and said his forces later helped remove the victims' bodies.
"We condemn the massacre by the FARC ... carried out against a peasant community that had nothing to do with the conflict," Salvatore Mancuso told local radio from a safe haven in northwest Colombia where his group is pursuing peace talks with the government.
Paramilitary militias have been blamed for some of the worst massacres in Colombia's civil war, which kills an estimated 3,500 people every year.
The FARC has spurned calls to declare a cease-fire and enter negotiations, though the government and the country's smaller rebel group, the National Liberation Army, or ELN, are currently trying to find common ground for peace talks.
By Kim Housego