Between September 26 and December 26 this year alone, "14 ex-members of FARC were murdered," said UN secretary-general Antonio Guterres in his quarterly report on the global body's mission to Colombia.

According to the state's special investigation unit, quoted by the UN, those responsible for the killings "are illegal armed groups and criminal organizations."

Most of those cases have been linked to the Gulf Clan drug-trafficking group that emerged out of disarmed right-wing paramilitaries in 2006, as well as FARC dissidents, the ELN guerrilla group and remnants of the now disbanded EPL Marxist rebels.

FARC is the Spanish acronym for the Revolutionary Armed Forces of Colombia; ELN is the National Liberation Army; and EPL is the People's Liberation Army.

- Guterres 'hugely concerned' -

The UN called on right-wing President Ivan Duque, a vocal critic of the peace accord signed by his predecessor Juan Manuel Santos, to "reinforce the security plans and strategies for ex-combatants."

In the report, Guterres said he was "hugely" concerned about the number of murders of social leaders and human rights defenders, saying the UN had verified 163 of 454 reported cases since the peace accord was signed.

"Most of the murders were in zones abandoned by former FARC (fighters) and where there is limited state presence," the UN report said.

Colombia's human rights ombudsman estimates that 423 activists were murdered between 2016 and the end of November.

Transformed into a political party since the peace deal, FARC has hit out repeatedly at the lack of security guarantees for its members.

While some 7,000 ex-fighters laid down their weapons, Colombia's peace and reconciliation commission estimates there remain 1,600 dissident rebels.

Colombia has been torn by more than a half century of armed conflict involving guerrillas, drug-traffickers, paramilitaries and state forces, leaving eight million people dead, disappeared or displaced.