Dissidents from Colombia's once largest rebel movement who were unable take advantage of a peace accord or grew disenchanted with the state's weak implementation are fueling violence in parts of the South American nation still waiting to see the benefits of peace, a rights group warned Thursday.

An investigation by Human Rights Watch found that former guerrillas are waging a brutal campaign of violence in the port city of Tumaco, underpinned by booming coca production, a lack of basic infrastructure and disillusionment with peace.

The group documented more than 120 crimes, including homicide, disappearances and rape in Tumaco, most taking place after the 2016 signing of the accord to end Latin America's longest-running conflict.

The homicide rate in Tumaco was four times higher than Colombia's national average in 2017 and ex-combatants with the former Revolutionary Armed Forces of Colombia are believed responsible for much of the violence, the group said.

"This is brutal," said Jose Miguel Vivanco, Human Rights Watch's Americas director.

Kidnappings and homicides have dipped to record lows in many parts of Colombia but residents live in fear in more remote regions where even basic services like safe drinking water and medical care are scarce.

Vivanco noted that at least 300 members of a criminal gang known as "Los Rastrojos" had become employees of the FARC before the peace deal but were not counted as former rebels when it came time to implement the accord. Instead many ended up joining dissident factions of the guerrilla army and returning to combat.

Other former rebels went to transition zones set up by Colombia's government only to find the sites lacked essentials like running water and electricity and eventually became so disillusioned they decided to join their dissident comrades.

Violence is being used in Tumaco by the dissident guerrilla front and other armed criminal groups competing for territory in one of the nation's biggest coca growing regions, Vivanco said.

Human Rights Watch documented 21 killings there, including that of a fisherman who was found shot to death with a sign on his chest that read "for thieving and snitching."

Vivanco said a military response alone would not be sufficient to quell the violence and urged Colombia's government to pursue development strategies that would improve economic prospects for residents and curb the illegal drug trade.