The ELN, Colombia's last active guerrilla group, acknowledged killing an indigenous leader, a violation of a historic ceasefire agreement with the government as the two sides hold peace talks.

"We deeply regret the incident and apologize for this painful case to his family and loved ones," the National Liberation Army (ELN) said in a statement Sunday about the death of indigenous governor Aulio Isarama Forastero in the northwestern department of Choco. Isarama had been detained by the guerillas under suspicion of links with "military intelligence."

"On the way to the interrogation, Governor Aulio Isarama Forastero refused to walk and rushed at one of our guerrillas, with the resulting tragic outcome," the statement said of the death last Tuesday.

It added that the ELN would carry out "an exercise of reflection at all internal levels so that events like this one do not happen again."

Isarama's death was the first violation of the temporary ceasefire that went into effect Oct. 1 and is meant to last until January 9, as the Colombian government and ELN leaders hold peace talks in the Ecuadoran capital Quito. The 1,500-strong ELN has been in negotiations with the government since February.

The government's chief negotiator, Juan Camilo Restrepo, wrote on Twitter that the killing is "deplorable from every point of view and disappointing."

The ELN and the 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."