BOGOTA (Reuters) - Colombia's Marxist rebel group, the National Liberation Army, will free a key hostage once delayed peace talks with the government begin, a commander said on Friday, challenging Bogota's demand it would not sit down at the negotiating table until the man is freed.

Peace talks between the government of President Juan Manuel Santos and the group, known as the ELN, were set to open Thursday in Quito, Ecuador, but Santos called off the ceremony pending the release of former legislator Odin Sanchez, held since April.

The release of Sanchez - the most well-known of potential remaining hostages - is a longstanding government condition for beginning talks with the group, the country's second-biggest insurgency.

"The commitment was that the liberation would take place in the course of the first round of negotiations," ELN negotiator Pablo Beltran said in comments posted to the ELN's Twitter account. "A date was not set."

"That's our commitment and we will follow through," Beltran said.

The ELN negotiator's comments seemed to contradict earlier reports that Sanchez's release was already under way.

The International Committee of the Red Cross, which will facilitate Sanchez's release, said on Thursday that protocols were in motion.

The head of the government negotiating team, Juan Camilo Restrepo, also said the operation had begun.

The 2,000-strong ELN, considered a terrorist group by the United States and European Union, has kidnapped hundreds of people during its 52-year insurgency to raise war funds and to use hostages as bargaining chips with the government.

Founded by radical Catholic priests and inspired by Cuba's 1959 revolution, the ELN has been in on-and-off, closed-door talks with the government since 2014 on how the two sides would conduct negotiations.

The government is scrambling to salvage a deal with larger rebel group the Revolutionary Armed Forces of Colombia, or FARC, after a peace accord was narrowly rejected in a referendum.

(Reporting by Julia Symmes Cobb, editing by G Crosse)