BOGOTA (Reuters) - Colombia on Friday put into action a military unit aimed at combating illegal armed groups that have begun to seize areas once controlled by Marxist FARC rebels in strategic drug-trafficking territory in the south of the Andean nation.

The so-called Hercules task force, the largest military unit activated in two decades, will have 9,000 troops with the mission of regaining control of a broad region bordering the Pacific Ocean and Ecuador.

In southern Narino province, which concentrates the majority of coca leaf crops - the raw material that makes cocaine - there are hundreds of former members of the Revolutionary Armed Forces of Colombia (FARC) who decided not to adhere to the peace accord signed late in 2016.

Scores of crime gangs and right-wing paramilitary groups jostle with the dissident FARC for control of the drug trafficking routes to the Pacific.

“The FARC has ceased to exist, now all this deployment, all our force must be directed against these organizations that still threaten the tranquility of the country,” President Juan Manuel Santos said at the launch ceremony in the city of Tumaco.

Military control of the area, which will be accompanied by investment to reduce poverty and coca eradication and substitution programs, is essential to guarantee a stable peace with the FARC and prevent other armed groups from strengthening, Santos said.

Santos, who leaves office in August after two terms, signed a peace agreement with the FARC in 2016 after four years of negotiations in Cuba. The FARC is now a political party with a presidential candidate for the May elections.

The task force will include officials from the army, the police, the air force and the navy, Santos said.