The Colombian army said Monday it had killed an ELN rebel and captured four others in an operation days before peace talks between the leftist guerrillas and the government are set to open.

The operation targeted the public order and financial wings of the National Liberation Army (ELN) in the northeastern department of Casanare, the military said in a statement.

The captured rebels included three commanders, one of whom was wounded, it said.

A separate operation in the western department of Choco resulted in the "demobilization" of 24 guerrillas, the defense ministry said on Twitter.

The ELN did not immediately comment.

The operations could deal a blow to peace talks due to open in the Ecuadoran capital Quito on October 27 between the rebels and President Juan Manuel Santos's government.

Santos won the Nobel Peace Prize this month for his efforts to end a half-century conflict in Colombia.

But he is struggling to save a peace deal with a larger rebel group, the Revolutionary Armed Forces of Colombia (FARC), after voters rejected it in a referendum.

The FARC and the government have been observing a ceasefire since August 29.

Santos extended it last week until the end of the year while the two sides seek a new deal more palatable to opposition voters who want to see former rebels do jail time.

But there is no ceasefire in place with the ELN.

"The two sides are going to keep fighting each other" as long as that is the case, said political analyst Carlos Medina of Colombia's National University.

The latest operations were not likely to jeopardize peace negotiations, he added, but urged the ELN to declare a unilateral ceasefire to reduce tensions in the run-up to the talks.

There had been signs of growing trust between the government and ELN in recent days.

The rebels freed three civilian hostages and vowed to release the two others they are still holding before the talks.

And the government agreed to free a group of imprisoned guerrillas so they could take part in the peace negotiations.

The FARC and ELN have been at war with the state since 1964.

The ELN is estimated to be about a quarter the size of the FARC, with some 1,500 fighters.

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