The 2016 Nobel Peace Prize laureate was speaking at a climate conference in the northern Colombian city of Santa Marta, two days after the country's worst attack on civilians in decades.

Saturday's highway bombing in the southwestern Cauca department -- which the government blamed on guerrillas opposed to a decade-old peace process -- killed 21 people in the run-up to next month's presidential elections.

Santos, who received a Nobel for negotiating a landmark peace with the Marxist rebel army FARC, held his successors' "lack of a coherent security policy" responsible for the current uptick in violence.

"Violence is returning in many regions due to the lack of implementation of the peace agreement, the lack of a coherent security policy," he told AFP.

"Criminal gangs are taking advantage of this situation, including fighting among themselves. That has generated much of the violence."

- Environment pays the price -

The 74-year-old, who wore shoes emblazoned with a peace symbol and a marijuana leaf in a nod to both peace and nature, said that "Without a doubt...tackling deforestation should be one of the main priorities" of any future peace agreement.

In the decade since the 2016 deal, deforestation across Colombia has shot up, with areas under guerrilla control heavily affected.

Santos blamed organized crime groups and their use of cattle ranching and illegal mining -- which both result in deforestation -- to finance their exploits.

He emphasized that peace and environmental preservation were "closely linked" causes, referring to conversations he had with Indigenous peoples living in and around Santa Marta.

"When I went to tell them, 'We have already made peace with the oldest and most powerful guerrilla in the Western Hemisphere (FARC), and now we are going to make peace with nature,' they told me, 'Yes, but make real peace, because peace with nature is necessary for peace among human beings."

He voiced hope that the conference, attended by some 60 countries looking to pivot away from fossil fuels, would lead to a "fairer, faster transition towards renewable energy."

The 2016 peace agreement ended a half-century of war between the government and FARC, but remaining armed groups continue to control large swaths of territory in remote parts of the northeast and south.

Concerns that violence is on the rise once again have dominated campaigning ahead of the May 31 presidential election.