Colombia's ELN guerrilla group says a Russian-Armenian citizen it held hostage for six months was killed in April while trying to escape, a startling admission that risks throwing current peace talks with the government into jeopardy.

In a rare interview, a commander of the National Liberation Army, Colombia's last active guerrilla group, said that ransoms from kidnappings were necessary to keep its fighters in the field and that peace would be impossible without state funding to feed and clothe the rebels.

The ELN seized Arsen Voskanyan in November. The group claimed that he was collecting endangered, poisonous frogs in the jungles of the northwestern department of Choco and accused him of wanting to smuggle wildlife overseas.

After his lengthy captivity, Voskanyan was shot when he grabbed a hand grenade in a bid to escape, according to the ELN commander, who would only give his nom-de-guerre Yerson.

"He's dead," Yerson told Reuters in a remote area along the banks of a river that sees frequent combat between the leftist rebels, government troops and right-wing paramilitaries.

"The grenade exploded … several of our boys were wounded, the entire unit of five boys. He fled, he was shot and killed … The issue of his body will be negotiated," he said, adding that the death took place within his unit. Yerson supplied no evidence to back up his assertions.

The ELN's practice of kidnapping civilians is a key issue at peace talks taking place in the Ecuadorean capital of Quito. The fact that Voskanyan was killed as talks progress and the ELN failed to inform the government may complicate already tricky negotiations to end 53 years of war with the Marxist group.