Colombia opens peace talks Tuesday with its last active rebel group, the ELN, seeking to replicate its historic accord with the FARC guerrillas and deliver "complete peace" after 53 years of war.

But experts warn the ELN will be a tougher negotiating partner than the FARC, and say no deal is likely before President Juan Manuel Santos -- the man who has staked his presidency on ending the conflict -- leaves office next year.

Santos, who won the Nobel Peace Prize in October, was nevertheless full of optimism heading into the talks.

"This conflict is over," he said Thursday.

"The public phase of negotiations between the Colombian government and the ELN... will enable us to achieve complete peace."

The Cold War-era conflict, which has killed more than 260,000 people and left 60,000 missing, is the last major armed conflict in the Americas.

Colombia, South America's third economy and the world's biggest cocaine producer, has been torn since the 1960s by fighting that has drawn in multiple leftist rebel groups, right-wing paramilitaries, drug gangs and the army.

Last November's landmark peace accord with the oldest and largest rebel group, the Revolutionary Armed Forces of Colombia (FARC), after four years of talks leaves the National Liberation Army (ELN) as the last active guerrilla insurgency.

It has an estimated 1,500 fighters, mostly in the north and west.

- 'More fundamentalist' than FARC -

The talks in the Ecuadoran capital Quito come after three years of secret negotiations and an embarrassing false start last in October, when the ELN refused to release their most high-profile hostage: ex-lawmaker Odin Sanchez.

A flurry of behind-the-scenes negotiations followed, leading to Sanchez's release on Thursday in exchange for two ELN prisoners.

In a further goodwill gesture on Monday, the ELN released a soldier they had captured two weeks earlier.

But there will be more bumps in the road, warned Frederic Masse, an expert on the conflict at the Universidad Externado in Bogota.

"The ELN has more fundamentalist demands than the FARC," he said.

"They want much deeper social change."

A prominent ELN commander warned ahead of the talks that the rebels would not back down on the thorny question of land rights for the rural poor -- one of the main issues in the conflict.

"As long as the necessities that were at the root of this insurgency exist, we will have to keep fighting," Danilo Hernandez, commander of the Resistencia Cimarron guerrilla front, told AFP in an interview.

- Complications: kidnappings, elections -

The talks are due to open at 5:00 pm (2200 GMT) at the Hacienda Cashapamba, a Jesuit retreat some 30 kilometers (20 miles) outside Quito.

The chief negotiators for the government and the ELN, Juan Camilo Restrepo and Pablo Beltran, will officially open the peace process before some 150 guests, plus journalists from around the world.

Negotiators will then get down to business on Wednesday, behind closed doors.

Despite Monday's hostage release, the issue of kidnappings remains a touchy subject.

Unlike the FARC, "the ELN has still not renounced kidnapping," long a source of revenue for both rebel groups, said Kyle Johnson of the International Crisis Group.

"They might kidnap someone else in the future and we'll be back in the same difficulties."

Elections in 2018 to decide Santos's successor also threaten to complicate matters.

The peace process faces ongoing resistance from conservative opponents who accuse Santos of granting impunity to rebels guilty of war crimes.

Santos had to tweak the initial FARC accord after voters narrowly rejected it in a referendum last October -- a major embarrassment for the government.

The slightly revised version was ratified in Congress, where Santos enjoys a majority.

A new poll found Tuesday that Colombians are growing less optimistic on the chances for peace. Polling firm Datexco, which interviewed 900 people nationwide on the prospects for peace, found 51.7 percent were optimistic, down from 67.4 percent in October.