Colombia's government and the country's second largest guerrilla group, the National Liberation Army, announced Tuesday they have opened peace talks, with a tense presidential election just days away.

In a joint statement released by the government, the two sides said they have been holding exploratory talks since January "with the objective of agreeing on the agenda and design of the process to make viable the end of the conflict and build the peace."

The government already is in the midst of peace talks with the Revolutionary Armed Forces of Colombia (FARC), Colombia's largest rebel group, which were launched in November 2012.

No date or place was given for the next set of talks with the National Liberation Army, or ELN, though Ecuador's president Rafael Correa said Tuesday his country was ready to "provide the facilities."

President Juan Manuel Santos said they would be no different than those being held with the FARC.

The process would involve the laying down of arms, an agenda with specific points, and talks held outside the country, he said.

"The political responsibility for this new peace effort falls exclusively on my shoulders and on no one else's," Santos said.

"A peace process that includes both the FARC and the ELN is the best guarantee ... that this conflict is ended for good, and that it will never be repeated."

The joint statement said the two sides have agreed to discuss the victims of the conflict and the rebel group's "participation in society."

"The rest will have to be agreed on" at a later time, the statement said.

UN chief Ban Ki-moon said he welcomed the start of talks with the ELN.

"Combined with the progress already achieved in the ongoing talks with the Revolutionary Armed Forces of Colombia (FARC), the Secretary-General hopes that this new development will contribute to ending the hemisphere's longest armed conflict and to building sustainable peace for the Colombian people," he said in a statement.

- Santos facing re-election battle -

The surprise announcement comes as Santos, who is seeking a second term, finds himself in a close run-off election on Sunday.

Opposition candidate Oscar Ivan Zuluaga has made the peace talks the main point of contention in the race.

He called the timing of the announcement of talks with the ELN "suspicious," saying it showed the "desperate attitude" of the government to influence voters.

The FARC, meanwhile, welcomed the ELN's talks with the government, calling the announcement "very important news for peace."

"We consider them (the talks) positive and we hope that they proceed in a spirit of peace and not in an electoral spirit," FARC negotiator Andres Paris told AFP in Havana.

Both sets of talks aim to end a leftist insurgency that has afflicted Colombia for half a century, claiming the lives of more than 220,000 people and uprooting another five million.

The FARC has between 7,000 and 8,000 fighters, according to the army. The ELN has another 2,000 or so combatants under arms.

As in the talks with the FARC, several countries will act as guarantors of the peace process, including Cuba, Chile, Ecuador, Norway and Venezuela.

"The message is one of a new opportunity, a historic event," said Fabrizio Hochschild, the head of the UN delegation in Colombia.

"It's very important because it completes the panorama of peace," said Jorge Alberto Restrepo, an analyst with the Colombian think tank Cerac.

"One could not have an internal conflict that is resolved with the FARC but continues with the ELN."

- Third time lucky? -

It is the third attempt at a negotiated peace with the ELN, after two failed efforts under the governments of Cesar Gaviria (1990-1994) and Alvaro Uribe (2002-2010).

After the government embarked on peace talks with the FARC -- their fourth attempt -- the ELN's leadership said it was ready to join the peace process.

Operating mainly in the north of the country along the border with Venezuela, the ELN recently focused on fighting "the looting of natural resources" by multinational companies.

It has pursued a campaign of sabotage against oil and mining installations, and has taken engineers and other workers hostage.

Unlike the FARC, the ELN has not publicly renounced hostage-takings as a tactic.

The ELN is led by Nicolas Rodriguez Bautista, alias "Gabino."