Suspected Marxist rebels massacred 16 peasants, including women and children, in a remote area in lawless Arauca Province, police said on Saturday.
The attack by the Revolutionary Armed Forces of Colombia, or FARC, happened Friday night less than an hour before the New Year arrived in the village of Puerto San Salvador, 380km northeast of Bogota, Arauca police chief Colonel Rodrigo Palacio said.
The FARC had accused the peasants of collaborating with right-wing paramilitary militias, Palacio said. Among the dead were six women and four children, he said.
A witness had originally told police that 17 people were killed, but when police arrived, they found that 16 were dead. It took police a while to reach the area, which is only accessible by boat.
The FARC, a smaller leftist rebel group and the outlawed paramilitary fighters of the United Self-Defense Forces, or AUC, have long battled to take control of Arauca, one of Colombia's most violent provinces that is a strategic corridor for smuggling drugs and arms from across the border in Venezuela.
The province is also rich in oil and these illegal groups have become rich syphoning royalties paid to the Colombian government by multinational oil companies.
The AUC is involved in a peace process with the government and has demobilized more than 3,000 fighters this year. The FARC has shunned government offers to start negotiations.
The massacre came the same day that President Alvaro Uribe took the unprecedented step of extraditing a top FARC commander to the US on drug trafficking and terrorism charges. The move has raised fears of reprisal attacks.
Colombia's armed forces chief put his troops on high alert after Ricardo Palmera, a former FARC peace negotiator believed to have been heavily involved in the group's financial operations, became the first FARC leader to be sent for trial in a US federal court.
The extradition came after the FARC failed to comply with an ultimatum from Uribe to free 63 hostages, including three Americans.
Colombia's 40-year-old conflict kills more than 3,000 people every year.