BOGOTA (Reuters) - A unit of Colombia's FARC rebel group said it will not lay down arms or demobilize under a potential peace deal with the government, the first public sign of opposition to an accord from within the rebel ranks that may set back efforts to end five decades of war.

The statement by the Armando Rios First Front, a 200-strong guerrilla unit in the southeastern jungle province of Guaviare, comes nearly two weeks after leaders of the Revolutionary Armed Forces of Colombia (FARC) and the government announced a ceasefire deal at their more than three-year-old peace talks.

"We have decided not to demobilize, we will continue the fight for the taking of power by the people for the people, independent of the decision taken by the rest of the members of the organization," the unit said in a statement on Wednesday.

The First Front, which famously held ex-presidential candidate Ingrid Betancourt and three American contractors hostage, said the deals being reached at talks in Cuba will not solve the social and economic problems which first motivated the Marxist group to take up arms in 1964.

President Juan Manuel Santos has said the peace talks, aimed at ending a conflict which has killed more than 220,000 and displaced millions, may conclude as early as this month. Any deal will be put to Colombians for approval in a plebiscite vote.

The First Front, which is known to have links to the drug trade, said it would respect any FARC rebels who agree to peace, but called on other units to join forces to continue the fight.

"We invite all guerrillas and militia who are not in agreement with demobilization and the laying down of FARC weapons to join forces and continue united as an organization," the statement said.

Santos said earlier on Wednesday that any FARC unit that does not adhere to a peace agreement would continue at war and be killed or jailed.

"Anyone with doubts, best leave them aside and join the peace accord, because it's the last opportunity they will have to change their lives, because otherwise they will end up, I assure you, in a grave or jail," Santos said.

FARC leaders negotiating in Havana did not immediately respond to the decision by the breakaway unit, but security sources said other units could also reject a peace agreement, and throw the process into doubt.

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