"No incident in itself will cause the breakdown of the ceasefire unilaterally or automatically," the office of Colombia's High Commissioner for Peace said in a statement.

The "serious incident" would be investigated by a verification commission comprising the UN, Catholic Church and both sides involved in ongoing peace talks, it said.

The National Liberation Army, or ELN, Colombia's last active guerrilla group, acknowledged killing the indigenous governor Aulio Isarama Forastero in the northwestern department of Choco last week.

In a statement released Sunday, the group said it had been holding Isarama under suspicion of links with "military intelligence" and apologized to his family and loved ones for killing him.

It said he was killed after he attacked a guard "with the resulting tragic outcome."

Isarama's death was the first violation of the temporary ceasefire that went into effect on October 1 and is meant to last until January 9, as the Colombian government and ELN leaders hold peace talks in Ecuador's capital Quito.

The 1,500-strong ELN has been in negotiations with the government since February.

Government chief negotiator Juan Camilo Restrepo wrote on Twitter that the killing is "deplorable from every point of view and disappointing."

The ELN and the Revolutionary Armed Forces of Colombia (FARC), Colombia's biggest guerilla group, were formed in 1964 to fight for land rights and to protect rural communities.

The conflict that raged for more than a half-century drew in leftist guerrillas, right-wing paramilitary groups and state forces and left 260,000 people dead, more than 60,000 missing and seven million displaced.

The ELN ceasefire came after a separate accord that saw the disarmament of the FARC.

The FARC has since launched a political party called the Common Alternative Revolutionary Force that will field candidates in next year's general elections.

Successful talks with the ELN would seal what Colombian President Juan Manuel Santos calls a "complete peace."