Historic peace talks between the Colombian government and ELN rebels have been thrown into doubt after the guerrillas were ordered to release their last hostage before negotiations could start.

The government's lead negotiator, Juan Camilo Restrepo, drew a furious response from the National Liberation Army (ELN) Monday after he issued an ultimatum to the leftist group: Free the hostage or the talks are off.

The ELN, which has pledged to release all its hostages before the meeting opens in the Ecuadorian capital of Quito on 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 have 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.

The ELN accused the government of "torpedoing" negotiations, raising doubts about whether the talks will get under way as planned.

"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 a quarter the size of the FARC, with about 1,500 fighters.

Sanchez 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.

The Sanchez family said they have not received confirmation of the former congressman's release but advocated continuing the dialogue as scheduled.

"For me, an ultimatum is always harmful," Patrocinio Sanchez told AFP.

"What we want is to have him back and this could harm my brother."

The International Committee of the Red Cross, which usually facilitates hostage releases, said it has not yet received a request to provide services "for humanitarian purposes as a neutral and independent intermediary".

President Juan Manuel Santos is racing to save a peace deal with the FARC after voters rejected an initial version in a referendum on October 2.

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, which has killed more than 260,000 people, left 45,000 missing and uprooted nearly seven million.