Colombia's government and FARC rebels must reach a new political deal quickly to prevent the peace process from unraveling, the UN envoy warned Wednesday.

The historic peace accord that ended the decades-old conflict suffered a setback when it was rejected in a referendum earlier this month, sending both sides back to the drawing board for fresh negotiations.

Colombian President Juan Manuel Santos has said he will extend a ceasefire with the FARC until December 31 and hopes to have a new agreement before that date.

"We need an agreement fairly quickly because while we are absolutely convinced that the two sides at the highest level have made a decision to end the war, things can start to unravel," Jean Arnault, the UN envoy for Colombia, told a news conference.

"We all know we need to move towards a political agreement as soon as possible," he said, urging a "serious effort" to reach out to those who voted 'no' in the referendum.

Arnault stressed that while there was strong agreement by all sides that there should be no return to war, the referendum outcome had injected uncertainty in the process.

The envoy, who also heads a new UN mission to monitor the ceasefire, said the deployment of monitors would continue to reinforce peace efforts.

"There is a sense of uncertainty so further deployment of the UN mission to observe the ceasefire will be an important factor," he said.

The Security Council on Tuesday agreed that the UN mission could continue to monitor the ceasefire after both sides made that a request and asked Secretary-General Ban Ki-moon to come up with recommendations on that mechanism.

Arnault said Ban will recommend that the mission continue its verification work, but it will not begin work on disarmament of the FARC rebels until a new agreement is in place.

About 400 UN monitors will be deployed in some 50-60 sites throughout the country.

Under the peace accord that was rejected in the referendum, the FARC's estimated 7,500 fighters are to disarm under UN supervision.

Santos launched talks with the Revolutionary Armed Forces of Colombia (FARC) after taking office in 2010, with the two sides sealing a historic deal on August 24 to end the conflict, which has claimed 260,000 lives.