The Cuban-inspired ELN began fighting the government in 1964
The leader of Colombia's second-largest rebel group has appealed to the country's largest, the Farc, to end fighting between them.
"We must order a stop to fratricidal war between our two forces," wrote the National Liberation Army (ELN) chief Nicolas "Gambino" Rodriguez.
He sent the message to Alfonso Cano, the new head of Revolutionary Armed Forces of Colombia (Farc).
The ELN has about 1,500 fighters but its organisation is under pressure.
The BBC's Jeremy McDermott in Colombia says many ELN units now ignore orders from their leader.
Others have made alliances with drugs trafficking gangs to ensure their survival economically and in the face of attacks by the Farc, our correspondent adds.
Both rebel groups have been forced onto the back foot by the US-supported offensives of President Alvaro Uribe, he adds.
Both the Farc and the ELN have been fighting the Colombian government since the 1960s and are believed to hold hundreds of hostages.
Meanwhile, Gen Freddy Padilla - the current head of the armed forces - has been appointed the new Defence Minister. He will retain his military post.
Gen Padilla replaces Juan Manuel Santos who recently resigned to prepare for a possible run for president next year.