The leader of Colombia's United Guerrillas of the Pacific (GUP), a group of former Marxist rebels who refused to demobilize under a peace deal, was killed in combat with the armed forces President Ivan Duque said on Saturday.

Victor David Segura, alias David, was killed near the city of Tumaco, in southwestern Narino province, a hub of coca plantations and fighting among armed groups for control of drug trafficking routes on the Pacific Ocean.

"Early this morning the most dangerous kingpin in the Pacific, known by the alias David, was neutralized. He was considered the head of the so-called United Guerrillas of the Pacific," Duque said during a community event in Amaga, in Antioquia province in northcentral Colombia.

The government had offered a reward of close to $50,000 for information leading to the capture of Segura, whose sister was also killed in the shoot-out with the Navy and police.

He led about 120 former Revolutionary Armed Forces of Colombia (FARC) fighters who remained in the jungle and continued murders, kidnappings, drug trafficking and other crimes rather than give up arms and participate in reintegration programs agreed in a 2016 peace deal to end more than five decades of war.

Some 12,000 FARC members, including 6,000 combatants, demobilized under the accord and the group is now a political party. However, more than 1,200 former FARC fighters did not demobilize - founding new groups dedicated largely to drug trafficking and illegal mining.

Other dissident former FARC fighters include the Oliver Sinisterra Front, which is accused of kidnapping and then killing two Ecuadorean journalists and their driver in April.