Colombian President Juan Manuel Santos says rebels belonging to the National Liberation Army have agreed to free a captive politician, clearing the way for formal peace talks with the country's second-largest rebel group to begin next month.

Santos made the announcement from the World Economic Forum in Davos, Switzerland. He said details of the agreement would be announced by representatives of the government and rebels at a press conference later Wednesday in Quito, Ecuador.

The government and the ELN have been holding exploratory peace talks for more than three years. Almost a year ago, the government announced formal negotiations would begin, but the start of talks has been held up by the rebels' refusal to meet Santos' demand they renounce kidnapping and free the politician.