The six people were traveling in a boat on the river Arquia when they were "illegally detained by armed men wearing military uniforms," Colombia's army said.

The kidnapping took place just two days after outgoing President Juan Manuel Santos admitted defeat in his bid to sign a ceasefire with the National Liberation Army rebels ahead of his handing over power to hardline right-wing successor Ivan Duque on Tuesday.

ELN Western War Front commander Omar Gomez confirmed his group was responsible in an audio recording sent to journalists.

"We will do everything possible to keep these gentlemen safe, giving them the best treatment we can," he said in the clip.

The two sides released a joint statement following the Cuban-hosted talks saying that "significant progress has been made."

On Wednesday, Santos had said there was "very little missing" from an agreement.

President-elect Duque however has called for a tougher stance on negotiations with the ELN, the last militant group fighting authorities in Colombia following the historic 2016 peace agreement signed with larger FARC guerrillas.

The army said the kidnapped soldier was on the boat suffering from a tropical disease, and was in was need of "urgent treatment."