By Helen Murphy and Peter Murphy
BOGOTA (Reuters) - Colombia's President Juan Manuel Santos has begun exploratory peace talks with leftist ELN rebels, the government said on Tuesday, just days before voters go to the polls in one of the tightest presidential election battles in decades.
The center-right government said it started discussing an agenda at early in the year to begin official negotiations to end a half century war with the National Liberation Army (ELN).
The government has been in peace talks with the ELN's larger counterpart, the FARC, since November 2012 in Cuba.
"We express to Colombia and the international community the reciprocal will to continue this exploratory phase which will allow us to agree on an agenda and establish talks to reach a final agreement," the government and ELN said in a joint statement that presidency posted on its website.
"The delegations agreed that the agenda will include points on victims and social participation."
Santos faces right-wing Oscar Ivan Zuluaga in a presidential ballot on Sunday, which is shaping up to be the closest race in two decades. Zuluaga has bitterly opposed the talks with the FARC and is likely to object to negotiations with the ELN.
"This shows that Santos, with whom we have profound differences, is doing something to bring about peace," said leftist lawmaker Ivan Cepeda, who helped facilitate the talks.
"Never has peace in Colombia advanced so much with the FARC and now there is this joint statement with the ELN. I hope this has some electoral result for Santos."
Colombia's leftist political parties have thrown support behind Santos in a bid to preserve the peace process. Cepeda said neither the FARC nor the ELN will continue peace talks if Zuluaga wins.
Zuluaga has said that if elected, he would not immediately end talks with the FARC but would impose conditions like jail terms and a ban on political participation.
The ELN has battled a dozen governments since it was founded in 1964 and is considered a terrorist group by the United States and the European Union.
Inspired by the Cuban revolution and established by radical Catholic priests, the ELN was close to disappearing in the 1970s but gradually regained strength. By 2002 it had as many as 5,000 fighters, financed by "war taxes" levied on landowners and oil companies.
The ELN is now believed to have about 3,000 fighters. It has sought peace before, holding talks with the Colombian government in Cuba and Venezuela between 2002 and 2007. Experts say there was a lack of will on both sides to agree a final peace plan.
(Additional reporting by Julia Symmes Cobb; Editing by James Dalgleish and Cynthia Osterman)