Colombia's President Juan Manuel Santos said that the National Liberation Army, or ELN, must release two hostages before a peace process pushes through.

The ELN, the country's second largest leftist group, is currently holding a civilian named Ramon Jose Cabrales, from the eastern Norte de Santander province, for five months, according to Reuters (via Channel NewsAsia). The government soldier is identified as Jair de Jesus Villar, who was captured by the rebel group last week in Antioquia.

"We demand the liberation of Corporal Villar and of citizen Ramon Cabrales," Santos said after his meeting with security officials in the city of Arauca, as quoted by the news outlet. "If they (ELN) want to begin any type of negotiation they must liberate these hostages."

The Colombian government and the ELN have been holding preliminary discussions for over two years while the former also negotiates a peace accord with the Revolutionary Armed Forces of Colombia, or FARC, the nation's most popular leftist group, Reuters further reported. The discussions with FARC taking place in Cuba are aiming to arrive at a final deal in the coming months.

FARC's head negotiator Ivan Marquez insisted that peace in Colombia would be "incomplete" if the ELN refuses to take part in the peace negotiations, Reuters added. The ELN bombed an army brigade in Arauca using six explosives on Monday.

The ELN, which has around 2,000 fighters, was founded in 1964 and is considered as a terrorist organization by the United States government and the European Union, Reuters noted. Despite the ongoing negotiation talks, the group has continued kidnapping and its attacks on infrastructure.

Santos said that he ordered the military "to intensify operations against the ELN and all the forms of delinquency that stem from its presence," BBC reported. The Colombian leader said that he issued the order after the group's attack in Arauca's military brigade.

"The ELN is dead wrong if it thinks that with attacks like this it can smooth the path to peace," the president said, as quoted in BBC's report. "If they think it will strengthen their position at the negotiating table, they are totally mistaken."

The attack on the army brigade damaged the compound's electricity supply, as well as buildings and cars, according to Colombia Reports. No one was injured during the attack.

Arauca, a city with 80,000 inhabitants, sits on Colombia's border with Venezuela, Colombia Reports added. The city, which is located at the midpoint of a territory controlled by the ELN, has long suffered from the armed conflict that came from the FARC and the ELN's efforts to overthrow the government.