NORTHWESTERN JUNGLES, Colombia (Reuters) - Colombia s ELN guerrilla group said 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. Another person with knowledge of the matter also subsequently confirmed that Voskanyan had been killed.  Reuters could not independently confirm the circumstances surrounding Voskanyan s death. Colombia s government said it knows nothing of the ELN s claim and the last it knew was a statement from the ELN that said he had escaped.  The responsibility is with the ELN,  the senior official said, asking not to be named. The Russian Embassy in Colombia, Colombia s High Peace Commissioner and the Foreign Ministry in Moscow did not immediately respond to requests for comment. 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 and make the need to agree a ceasefire more pressing.  It makes it urgent to get a bilateral, verifiable ceasefire as soon as possible so this doesn t keep happening,  leftist Senator Antonio Navarro Wolff, who once belonged to now-demobilized urban guerrilla group the M-19, told Reuters. Yerson and his troops said they are not optimistic a peace agreement can be reached because neither side will give ground on kidnapping. The ELN has refused to stop taking hostages for ransom, launching bomb attacks and extorting foreign oil and mining companies while talks are ongoing. The government has said it will not move forward on issues like a bilateral ceasefire until it does. Talks with the ELN are being held as the Revolutionary Armed Forces of Colombia (FARC), until this year the biggest rebel group, has demobilized, formed a new political party and ended its part in a civil war that killed more than 220,000 people and displaced millions over five decades. His face covered by a thin black balaclava and wearing a beret and camouflage fatigues, Yerson, 35, said he has been fighting in Colombia s jungles and mountains  for many, many years.   Flanked by two fighters carrying semi-automatic rifles as other rebels watched on, he questioned the government s willingness to make sufficient concessions but said he would adhere to the wishes of his leadership if a peace deal was reached. The ELN has sought peace before, holding talks in Cuba and Venezuela between 2002 and 2007, but experts have said those discussions were dogged by lack of will on both sides. Yerson is the commander of the Ernesto  Che  Guevara Front, that fights under the command of the ELN leader known as Uriel who commands the Western War Block Omar Gomez. He declined to say how many rebels fight in his unit. The ELN - which has kidnapped hundreds of Colombians and foreigners for economic and political gain - previously said in a statement that Voskanyan escaped injured after a struggle that left several fighters wounded as they tried to release him to the International Committee of the Red Cross. The killing of Voskanyan may turn already dire public perception further against the ELN, analyst Ariel Avila told Reuters.  The impact will be on public opinion and in the questioning of the talks,  he said. Inspired by the Cuban revolution and established by radical Catholic priests in 1964, the ELN was close to disappearing in the 1970s but steadily gained power again.  By 2002 it had as many as 5,000 fighters, financed by  war taxes  levied on landowners and oil companies. It is now believed to have about 2,000 fighters, but Yerson, who would not confirm the number, said the group is heavily recruiting.  Considered a terrorist group by the United States and the European Union, the ELN has stepped up attacks on economic infrastructure this year, hitting oil pipelines and power lines repeatedly. President Juan Manuel Santos, who meted out some of the most crushing military blows against the FARC and earned a Nobel Peace Prize last year for his efforts at peace, has had less success with the ELN, which moves in mobile units of four or so fighters.  The ELN has said it may declare a temporary ceasefire to honor Pope Francis during his visit next week to Colombia. 