The Colombian government is set to sit down for a first round of peace talks with the ELN rebel group, seeking to end a 53-year conflict that has killed more than 260,000 people.

The negotiations with the National Liberation Army (ELN) - the country's last active rebel group - mark a new milestone in the Colombian peace process, after President Juan Manuel Santos's government sealed an historic accord with the country's largest rebel group, the Revolutionary Armed Forces of Colombia (FARC), in November.

"The dialogue with the ELN fills us with optimism," Mr Santos tweeted, hours after a ceremony to open the talks in the host country Ecuador. "New generations and the victims deserve that the talks move forward and we arrive at a complete peace."

The small ceremony took place at a Jesuit retreat outside the capital Quito.

"We have before us the opportunity to finally turn the page on this war," said the government's chief negotiator, Juan Camilo Restrepo.

But experts warn the ELN will be a tougher negotiating partner than the FARC.

There was friction between the two sides even as they celebrated the formal opening of talks.

Mr Restrepo warned the rebels that if they fail to give up kidnapping, "it will be very difficult to advance."

Cashapamba estate in Ecuador where the peace talks are taking place

The ELN's chief negotiator, Pablo Beltran, for his part called on the government to "take responsibility" for its actions during the conflict - saying the rebels were prepared to do the same.

Colombia is the scene of the last major armed conflict in the Americas.

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

November's landmark peace accord with the FARC, the oldest and largest rebel group, leaves the ELN the last active guerrilla insurgency.

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

'More fundamentalist' than FARC

The talks come after three years of secret negotiations and an embarrassing false start in October, when the ELN refused to release its most high-profile hostage, the 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 rebels 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."

Complications: kidnappings, elections

The talks mark the fifth effort to make peace with the ELN, after a string of failed attempts in the 1990s and 2000s.

Negotiators will now get down to business behind closed doors today.

ELN guerrillas have been active for decades (file pic)

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, Kyle Johnson of the International Crisis Group said.

"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.

Mr Santos, who won the Nobel Peace Prize last year and who will meet President Michael D Higgins during his tour of Latin America next week, 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 the president enjoys a majority.

A new poll found that Colombians are growing less optimistic about the chances for peace, however.

The polling firm Datexco, which interviewed 900 people nationwide on the prospects for peace, found 51.7% were optimistic, down from 67.4% in October.