He is believed to be the last hostage held by the guerrillas, who freed several others in the weeks leading up to the talks
Colombia's ELN rebels accused the government Monday of "torpedoing" peace talks due to open this week by insisting the leftist guerrillas free their last remaining hostage.
Three days out from the historic talks, tension flared when the government's lead negotiator, Juan Camilo Restrepo, gave the National Liberation Army (ELN) an ultimatum: Free the hostage, or the talks are off.
The ELN, which has pledged to free all its hostages before the talks open Thursday, is still holding former congressman Odin Sanchez in the remote jungle region of Choco, along the Panamanian border.
He is believed to be the last hostage held by the guerrillas, who freed several others in the weeks leading up to the talks.
"If Odin Sanchez isn't released safe and sound between now and Thursday, the conditions will not be in place to begin the public phase of the negotiations," Restrepo said in an interview on Caracol Radio.
That drew a furious response from the ELN.
"J.C. Restrepo's statement torpedoes the joint actions that must be taken by (Thursday)," the insurgent group wrote on Twitter.
The ELN is Colombia's second-largest rebel group, after the Revolutionary Armed Forces of Colombia (FARC) -- which is also holding peace talks with the government.
The ELN is estimated to be about one-fourth the size of the FARC, with some 1,500 fighters.
President Juan Manuel Santos is currently racing to save a peace deal with the FARC after voters rejected an initial version in a referendum.
The deal was the product of nearly four years of talks in the Cuban capital Havana.
Santos, who won this year's Nobel Peace Prize for his efforts, has staked his legacy on ending half a century of conflict in Colombia.
The conflict has killed more than 260,000 people, left 45,000 missing and uprooted nearly seven million.
The ELN talks are due to be held in the Ecuadoran capital Quito.
Sanchez, the ELN's last hostage, handed himself over to the group in April in exchange for the release of his brother, Patrocinio Sanchez, a former governor who had fallen ill after nearly three years in captivity.