Dozens of rights activists were killed in Colombia last year, according to a report by the UN High Commissioner for Human Rights.
It says most victims were human rights leaders or members of left-wing groups, and calls on the government to provide protection.
Areas with illegal drug crops were the most dangerous, it said.
The UN warned that armed groups were moving into territories previously occupied by Farc rebels.
Under a peace deal signed in November, the Farc rebels have moved to transition zones where they will remain until they fully demobilise.
The UN said the illegal armed groups were fighting for territory and resources.
The UN representative in Colombia, Todd Howland, said the groups were often involved in drug trafficking and illegal gold mining and see activists as a threat.
Mr Howland called on the government to recognise that the deaths formed a pattern and to provide protection to those in danger.
Colombia's Interior Minister Juan Fernando Cristo, said violence against rights defenders could undermine the peace deal.
"We're all interested in facing these threats and these murders because we know that it seriously affects the chances of consolidating peace in Colombia," he said.