Three off-duty soldiers were kidnapped by suspected ELN guerrillas on Colombias northeastern border with Venezuela, authorities said on Thursday.

The news comes a week after Marxist ELN rebels offered to release six other hostages, including three policemen and a soldier, taken the day before in a jungle area.

The soldiers "were on leave, wearing civilian clothes and unarmed," the military said in a statement.

Colombia's ombudsman called for "respect for life, integrity and the release of the soldiers," on its Twitter account.

Having owned up to last week's kidnappings, the ELN demanded an end to military operations in the northwestern Choco department in exchange for the release of the prisoners, one of whom was suffering from a tropical disease and in need of "urgent treatment," according to the army.

The ELN, or National Liberation Army, has around 1,500 guerrillas and is the last recognized armed group fighting government forces in Colombia following the 2016 peace accord signed with left-wing FARC rebels.

Last week, ex-president Juan Manuel Santos admitted falling short in his bid to sign a ceasefire with the ELN before handing over power to Ivan Duque, who assumed office on Tuesday.

Hard-right Duque has called for a tougher stance in peace negotiations with the ELN while also vowing to "deploy corrective measures" to the FARC deal.