Diaz's mother Cilenis Marulanda and father Luis Manuel Diaz were taken by armed men as they were driving in La Guajira province on Saturday. Marulanda was freed within hours.

Colombia's government reported the ELN was responsible for the kidnapping earlier on Thursday. Interior Minister Luis Fernando Velasco said the situation was "very serious" and that it violated a ceasefire between the government and the rebels.

Peace talks between the ELN and the government restarted last year in hopes of ending the group's part in Colombia's 60-year conflict, which has killed at least 450,000 people.

The government and the ELN began a six-month ceasefire in August as part of talks.

Diaz's father will be released "as soon as possible," ELN representative Juan Carlos Cuellar told a community meeting, in a video clip shared by the ELN with Reuters.

Earlier on Thursday, the government's peace delegation charged with negotiating with the ELN called for Diaz's father to be released immediately.

"We remind the ELN that kidnapping is criminal, violates international humanitarian law and that its duty in building the peace process is not just to stop (kidnapping) but to eliminate it forever," Otty Patino, head of the government's peace delegation, said in a statement.

The ELN, Colombia's most radical leftist guerrilla group, has long funded its operations with kidnapping, as well as extortion and drug trafficking, according to security sources.