The Colombian government said Thursday it will postpone peace talks with the ELN rebels until they release an ex-lawmaker they are holding hostage, just hours from the scheduled start of negotiations.

Interior Minister Juan Fernando Cristo said the government was prepared to postpone the talks -- scheduled to start at 5:00 pm (2200 GMT) -- until it was sure former congressman Odin Sanchez had been freed.

"If the opening (of the talks) can't be held today, the government is ready to do it tomorrow or Saturday or whenever we are certain that Odin Sanchez has been freed," the minister told a press conference.

The comment came as government and rebel negotiators gathered in the Ecuadoran capital Quito, awaiting a green light for the formal opening of the talks.

The National Liberation Army (ELN) is Colombia's second largest rebel group, after the Revolutionary Armed Forces of Colombia (FARC), which has been in talks with the government for nearly four years.

The ELN talks are meant to open a new, decisive front in President Juan Manuel Santos's efforts to end an armed conflict of half a century that has killed more than 260,000 people.

Santos, the winner of this year's Nobel Peace Prize, has already signed a peace deal with the FARC, but voters rejected it in a referendum this month -- sending negotiators back to the drawing board.

The ELN talks are shaping up to be even trickier.

The ELN had promised to free its hostages before the negotiations.

But it bristled last week when the government's chief negotiator gave it an ultimatum to free Sanchez, accusing him of "torpedoing" the talks.

Like the FARC, the ELN formed in 1964 and is blamed for killings and kidnappings during a many-sided civil war.