Colombia's largest rebel group, the Farc, has urged the government to include the smaller ELN guerrilla group in peace talks aimed at ending more than half a century of conflict.
The Farc leader, known as Timochenko, said he considered the matter urgent.
On Monday, President Juan Manuel Santos said he had facilitated a meeting between the leaders of the Farc and the ELN.
The UN in Bogota welcomed the move.
But critics of President Santos' peace process say that by trying to include the ELN, Farc is using delaying tactics and that the group is reluctant to give up military conflict.
The peace talks have been going on for two years in Havana, Cuba.
Timochenko's comments were published on the Farc website.
He said: "We believe that not only for us as a revolutionary movement is it urgent and necessary that the ELN is brought into the peace talks. It is also urgent for the government and for the Colombian people. It is the right and practical thing to do.".
The government has been negotiating with the ELN in secret for more than a year.
Its leader - known as Gabino - has said that little remains to discuss before his movement agrees to enter formal peace talks.
"We have around 80% agreement (on the agenda), and we only have 20% which still has to be placed publically on the table," he told the Reuters news agency in a video message.
For both the ELN and Farc a key stumbling point is whether seeking a peaceful solution to the conflict would mean that their members would be offered amnesties or face prison sentences for war crimes.
"It is absurd to think we need to pay with prison sentences for looking for a political solution to the conflict," Gabino told Reuters.
The ELN was set up in 1964 and was inspired by the Cuban revolution. It launches attacks on the Colombian armed forces from its jungle bases.
It is estimated to number around 2,000 fighters.
The latest round of peace talks in Havana ended on 8 May with a call from the Farc to dismantle paramilitary groups and without an agreement over reparations for the victims of the conflict, an issue that has been debated for months.