Colombian President Juan Manuel Santos ordered his government on Friday to step up an offensive against the leftist ELN guerrilla movement after the rebels showed off a soldier's severed leg.

Santos expressed disgust at reports that the National Liberation Army had placed an anti-personnel mine in a children's park in northwestern Colombia and that the explosion blew off both of the soldier's legs.

Speaking during a visit to Mexico, Santos said placing mines "is already a savage act" but "showing a leg like a trophy next to a college, that's a barbaric act that borders on madness."

He said he ordered the armed forces "to intensify (the offensive) against the ELN" and "redouble efforts against this criminal organization."

While the government has held peace talks with the country's biggest rebel group, the FARC, since 2012, it has not launched any dialogue with the ELN.

Despite his tough talk Santos reiterated his willingness to begin peace negotiations with the ELN.

"The ELN is part of this war, part of this conflict, and it would be ideal for us to reach an agreement with both (rebel) groups," he told Mexico's Televisa channel.

The 2,500-strong ELN said in June that it had begun exploratory discussions with the government in a bid to launch negotiations in parallel with Bogota's talks with the FARC.

As for the FARC talks, Santos said there was a "more than 50 percent" chance that a final peace deal can be agreed to end a conflict that began in the 1960s.

On Friday, government and FARC negotiators, whose meetings have been hosted by Havana, agreed on a "roadmap" to remove landmines.

Colombia is second to Afghanistan in the world for landmine victims, with more than 11,000 people killed or wounded since 1990.