Nobel laureate Juan Manuel Santos on Wednesday said "a heavy hand alone" will not restore order in Colombia, days before a landmark peace deal's tenth anniversary.
The South American country is the world's largest cocaine producer and is dealing with its worst levels of violence in a decade.
President Abelardo de la Espriella, three months into the job, firmly opposes dialogue with armed groups and has vowed to send them all "to the grave or prison."
Santos, who served as president from 2010 to 2018, oversaw the historic peace deal that led to the rebel army FARC laying down arms after decades of fighting.
"If you want to regain security, you need to regain control of the territory... that isn't achieved just with a heavy hand, by bombing," he said during the presentation of an Ombudsman report.
Santos, who won the Nobel Peace Prize for his role in the 2016 agreement, urged De la Espriella to comply with its terms.
"Don't miss this opportunity," he said, adding that the accord had been "targeted by attacks and disinformation campaigns."
Another ex-president attending the presentation, Ivan Duque, criticized Colombia's special peace tribunal which tries cases involving the conflict's worst crimes.
Parts of the political right consider sentences handed to guerrillas as too lenient, and sentences handed to state-backed paramilitaries as too harsh.
Obudswoman Iris Marin noted that De la Espriella did not attend Wednesday's event.
"Without a doubt we miss the government, but they're part of the discussion even if they're not here," she said.
"No one wants to stop the war" in Colombia, Marin told AFP last week.
"The armed groups don't want to stop the war, and they've proven it. They are cruel, they haven't taken advantage of the opportunities for peace. And the national government, for that very reason, isn't going to stop the war either," she said.
The 2016 accord significantly reduced violence in one of Latin America's bloodiest conflicts, although remote areas of Colombia continue to suffer from the presence of illegal armed groups.
Some 521 ex-FARC signatories have been killed in the decade since the group disbanded, according to the Ombudsman report.
Former combatants are mainly killed by drug traffickers and rebels who broke away from the peace deal, it wrote.
Killings of ex-guerrillas rose by 38.5 percent last year, it said, at a time when the country is breaking its own world record for cocaine production rates.
Out of over 14,100 signatories, some 11,000 ex-combatants continue to comply with the terms of the 2016 agreement, it said.
The deal saw the originally leftist forces lay down arms in exchange for a seat at the political table, with guaranteed representation in Congress for eight years.
It also established alternative sentences to prison for those who confess their crimes, make reparations to their victims and pledge not to return to violence.
The Ombudsman warned against "undermining" the progress achieved in the country battered by a decades-long armed conflict.
The report also notes a 740 percent increase in forced displacement between 2017 and 2025.