Colombia's ELN fighters, currently in exploratory peace talks with the government, has claimed responsibility for abducting a mayor, saying he was corrupt.
The mayor of Alto Baudo town, in Choco province on the Pacific coast, was kidnapped on December 16 as he traveled on a river in the impoverished region.
In a message titled "trial of a corrupt Choco official," the National Liberation Army said "the goal is to force him to justify himself before the city administration."
"The mayor campaigned on promises to invest in health and education. But once he got in power, he did the same thing as other officials in the province: steal money from the city's coffers," the rebels wrote in a message posted on their website.
"He will be freed once he promises to return the money he stole."
President Juan Manuel Santos has called for the quick release of the mayor, saying his kidnapping was "unacceptable."
Colombia's second-largest leftist guerrilla group, which is believed to have about 2,500 fighters, said last month it was prepared to hold formal peace negotiations with the government aimed at ending a half-century old insurgency.
The ELN would be following in the footsteps of the larger Revolutionary Armed Forces of Colombia, which has been in peace talks with the government for more than two years.
Colombia's internal conflict, the longest-running leftist insurgency in Latin America, has killed some 220,000 people and displaced 5.3 million more.