Colombia's government hopes to open a new front Thursday in efforts to bury a half-century armed conflict, starting talks with the country's second-biggest rebel force.

An accord with the leftist National Liberation Army (ELN) was meant to be the icing on the cake of a historic agreement signed last month with Colombia's biggest rebel group, the FARC.

That was until voters surprised the government by rejecting the FARC accord in a referendum on October 2.

Now, President Juan Manuel Santos is trying to salvage the FARC accord while also talks with the ELN.

His government plans to formally establish negotiations Thursday in the Ecuadoran capital Quito with the ELN, Colombia's second-biggest insurgent group.

Like the FARC, the ELN formed in 1964 and is blamed for killings and kidnappings during a multi-sided 52-year civil war.

- Hostage dispute -

A dispute over the last hostage held by the ELN, former congressman Odin Sanchez, has threatened to delay the start of talks, however.

The government insisted the ELN free all its hostages before talks could begin -- just as it had at the start of negotiations with the FARC in 2012.

The ELN bristled this week after the government's lead negotiator Juan Camilo Restrepo issued an ultimatum for Sanchez's release.

But a Catholic Church spokesman close to the negotiations said on Tuesday that moves were under way to free him in time.

"All the protocol is being observed and the proceedings are on track," said Dario de Jesus Monsalve, archbishop of the city of Cali.

"The operation is under way and I hope they will make an effort to hand him over before October 27," he said on Caracol Radio.

The rebel force said on Twitter late Monday: "Overcoming difficulties, the ELN delegation for the peace talks is getting ready to be in Quito."

- ELN 'getting ready' -

Analyst Camilo Echandia of Colombia's Externado University said the ELN was reluctant to accept the release of hostages as a condition for talks, even though that would show a "will for peace."

"That is the big difference between the ELN and the FARC," he told AFP. "These negotiations are going to be very complicated."

Incidents involving ELN forces have kept tensions high over recent months.

The Colombian army blamed the ELN for a non-fatal explosion at an oil pipeline near the Venezuelan border on Sunday.

"The ELN guerrilla group comes strengthened to the negotiations with the government. Over the past three years this group has increased its level of violence," Colombia's Conflict Analysis Resource Center (CERAC) said in a report this month.

"This public phase of negotiations comes in the midst of the conflict, so the ELN will probably maintain a high level of violent action."

- Drugs and violence -

Colombian authorities estimate the ELN currently has some 1,500 members. The army says hundreds have deserted or been captured over recent months.

Its activities are restricted mainly to parts of the north and west of the country, according to CERAC.

It said it has seized tonnes of cocaine and marijuana from the ELN and destroyed drug laboratories under its control.

Colombia's territorial and ideological conflict has drawn in various guerrilla and paramilitary groups, drug gangs and state forces over the decades.

The conflict has killed more than 260,000 people and left 45,000 missing, according to Colombian authorities.