Colombia's main guerrilla group, FARC, on Monday rejected the government's efforts to put the peace deal both sides are finalizing up to a plebiscite, insisting a national constituent assembly should be called instead to determine the country's future.

Bogotá is hoping to pass a law no later than Dec. 16 that would impose such a referendum to give Colombian voters final approval over any peace plan, Reuters reports. But the Revolutionary Armed Forces of Colombia (FARC) announced in a statement their opposition to the initiative.

"We reject it not only because it is an arbitrary formula, but because it is the safest and least conducive of all the initiatives that are before the Congress of the republic as a result of the peace process," the militant group insisted.

But Sen. Roy Leonardo Barreras Montealegre, who heads President Juan Manuel Santos' Social Party of National Unity, told Colombia's El Nacional that the plebiscite might be held as early as March of next year and that the guerrilla groups were expected to disarm beginning in May.

"Colombians have great doubts about the process, and it is our obligation to clear them up, to explain to them how a peaceful country is more prosperous (and) that the country dedicate itself to the common safety, which is what is most important to the compatriots," Barreras said.

"These issues have been neglected because the heap of resources has been invested in a war that has only brought us dead bodies."

Under a self-imposed deadline, the government in Bogotá and the FARC leadership have until March 23, 2016, to sign a final agreement, Reuters reported. But in a symbolic gesture announced by rebel chief Rodrigo Londoño, the group has already suspended the purchase of weapons and munitions.

"On Sept. 30 I gave the order to suspend the purchase of arms and munitions to the entire (FARC) structure," said Londono, who is known by his nom de guerre "Timochenko."