Colombia vowed Monday to step up the fight against the country's second-biggest rebel group, despite claims it was ready to join in a peace drive to end decades of conflict.

President Juan Manuel Santos spoke after a meeting of security officials prompted by an attack early Monday on a military brigade, blamed on the leftist ELN, or National Liberation Army.

He said he had ordered the military "to intensify operations against the ELN and all the forms of delinquency that stem from its presence."

The attack raised tension amid efforts to include the ELN in peace efforts alongside the FARC, Colombia's biggest rebel force, aimed at ending half a century of conflict in the South American country.

"The ELN is dead wrong if it thinks that with attacks like this it can smooth the path to peace," Santos said.

"If they think it will strengthen their position at the negotiating table, they are totally mistaken."

The Colombian government has been holding peace talks for the past three years with the leftist FARC.

In 2014, he also launched efforts to convene formal peace talks with the ELN, without calling a ceasefire between the group and government forces.

Santos is demanding the ELN release two captives as a condition for any peace deal.

The FARC and the government have said they aim to sign a peace deal by March 23.

An agreement to start peace talks with the ELN has not yet been reached.

The FARC however said Monday it believed the ELN was ready to start negotiations "as soon as possible."

"The ELN cannot stay outside the peace process," the FARC's chief negotiator Ivan Marquez told reporters in Havana, where the negotiations are ongoing.

"We have grounds to assert that its leaders want to start peace discussions as soon as possible with the Colombian government. A peace without the ELN would be an incomplete peace."