By Luis Jaime Acosta

BOGOTA (Reuters) - Colombia's army will up their fight against dissident FARC rebels and crime gangs across the nation even after a peace accord is signed with the Marxist group, the head of the army said, in a bid to prevent other armed groups and drug traffickers from taking over rebel territory.

The government of President Juan Manuel Santos and the Revolutionary Armed Forces of Colombia (FARC) have been in peace talks for nearly four years, and recently signed a historic ceasefire deal considered the penultimate step to a final accord.

Experts have expressed concern that an end to the war with the FARC could lead to military withdrawal from rebel areas, leaving them open for takeover by smaller groups such as the National Liberation Army (ELN) or crime gangs, as well as dissident members of the FARC who refuse to disarm.

"Once they sign the final deal with the FARC we'll start a period that experts say could last a decade or more - battles against some groups that haven't joined the process like the ELN, and if there are FARC dissidents, or organized armed groups," General Alberto Jose Mejia told Reuters late on Wednesday.

Colombians are set to vote on whatever accord is agreed by negotiators, who still have to reach a deal on overall implementation and how a referendum could be organized.

A peace deal to end the conflict, which has killed 220,000 and displaced millions over more than five decades, will not silence military guns, Mejia said.

"We will not abandon the regions, we will not permit so-called ungoverned areas, we will not permit the arrival of other violent actors in zones the FARC leave," he said.

The FARC's First Front, a unit notorious for holding an ex-presidential candidate and three American contractors hostage, said in a statement released on Wednesday that it would not lay down arms under a peace deal. Santos has said those who do not demobilize will be jailed or killed in combat.

A FARC demobilization will allow the military to redirect its efforts against crime gangs, including groups who traffic drugs, the general said.

After a deal some 10,000 soldiers will form part of two de-mining brigades focused on removing explosives from thousands of square kilometers of territory, Mejia said.

Colombia is second only to Afghanistan and Cambodia in numbers of landmines victims, according to the United Nations. More than 2,200 Colombians have been have been killed and 9,100 injured by the devices.

(Reporting by Luis Jaime Acosta; Writing by Julia Symmes Cobb; Editing by Helen Murphy and Diane Craft)