Colombia might have managed to take significant strides towards reaching a peace deal with the country's FARC rebels, but the nation's number two guerrilla group, the National Liberation Army, or ELN, is proving to be a much more challenging group to handle.

Following a recent ELN attack against a military brigade in the northern state of Arauca, Colombian President Juan Manuel Santos has ordered the country's military to initiate a massive crackdown against the notorious rebel group on Monday. The attack had been initiated in an area where the rebels have been known to operate, reported UPI News.

The ELN and the Colombian government has been attempting to engage in preparatory peace talks for the past two years. However, unlike the FARC negotiations, the negotiations with the ELN have not had much progress, with both the rebel group and the Colombian government accusing each other of stalling.

With regards to the recent attack on Monday, Santos is firm about his position. Speaking to BBC News, the Colombian president asserted that such actions would not be tolerated.

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

Apart from condemning the attack, the Colombian president also demanded that the rebel group release two captives immediately, in order for formal peace talks to properly be initiated.

Even FARC negotiators themselves have stated that with the current steps the country is taking towards an ever-elusive peace, the ELN must participate in programs that are geared towards ending the conflict.

FARC's chief negotiator, Ivan Marquez, emphasizes that importance of the ELN's cooperation.

"The ELN cannot stay outside the peace process," he said.

The National Liberation Army was formally founded back in 1964, portraying itself as a powerful Marxist guerilla group which was created to fight against the country's proposed privatization of natural resources and the evident inequality of the distribution of land and wealth in Colombia.

Though commonly dubbed as Colombia's number two rebel group as a result of FARC's widespread influence and reach, ELN is nonetheless a formidable force, boasting about 2,000 active fighters to this day. It has been alleged that its fighters are distributed in the Norte de Santander, Arauca and Narino states, which are known strongholds of the rebel group.

The group has made a name for itself through acts of extortion, drug trafficking, and kidnappings for ransom.