Colombia's president, fresh off an announcement that the government and FARC rebels aim to sign a definitive peace deal within six months, encouraged the country's other remaining rebel group Sunday to follow suit.

"The National Liberation Army (ELN) cannot remain the last guerilla group in the Americas, fighting against a country's goodwill and desire for peace," President Juan Manuel Santos said in the newspaper El Tiempo.

The ELN and Colombian government have been in exploratory talks since January 2014, but have not yet opened a formal peace process.

"As I have said many times: Hopefully they will not let the train of peace pass by," Santos said, adding that the deal with the FARC "should motivate them."

On Wednesday, the Colombian government and FARC rebels revealed they had reached an agreement on the critical issue of justice for crimes committed during the country's 50-year conflict, which has killed more than 220,000 people and uprooted six million.

The justice issue had been the one preventing progress in the negotiations, which began in November 2012 in Havana.

Santos and FARC leader Timoleon Jimenez also vowed negotiations would conclude "at the latest" in six months.

The ELN has about 2,500 fighters, compared to the FARC's 7,000.