The United Nations refugee agency is urging the Colombian authorities to investigate the reported murder of 17 indigenous Awa people last week.
Unconfirmed reports said left-wing rebels targeted several families they accused of collaborating with the army.
Some 21,000 Awa live in the southwest province of Narino and the UN fears the killings will prompt a mass exodus.
The Awa are often caught in fighting between drug gangs and guerrillas as they battle for control of drug crops.
UNHCR spokesman Ron Redmond said initial reports suggested "an irregular armed group" carried out the attack on the Awa tribe in a remote jungle region in Narino, killing 17 people.
"The rest of the population is now extremely frightened," Mr Redmond told a news briefing in Geneva. He said there were increasing concerns that there could be a mass displacement of people in the days to come.
According to a witness who managed to escape, rebels believed to be from the Revolutionary Armed Forces of Colombia (Farc) rounded up several families suspected of working with the army.
They tied them up, beat them and then stabbed them to death, the witness was quoted as saying.
The area where the reported killings happened is remote and extremely difficult to reach, with many landmines in some parts.
The security forces and aid agencies are trying to reach the zone to verify the reports and deliver help.
The Awa have long pleaded with the government for protection, as not only the Farc but the smaller rebel group, the ELN, as well a several paramilitary drug-trafficking gangs operate on their land, says the BBC's Jeremy McDermott in Colombia.
The Awa are one of 87 different indigenous groups in Colombia, a third of which are at risk of extinction as a result of armed conflict and forced displacement, the UNHCR says.