Colombian President Juan Manuel Santos and FARC leader Rodrigo Londono vowed to keep working for peace Sunday after Colombian voters narrowly rejected the peace accord the leaders signed just days ago.

"I won't give up. I will fight for peace until my last day in office," Santos said in Bogota, flanked by the government team who hammered out the now-defunct deal over nearly four years of talks in Havana.

Santos said negotiators would return to Havana Tuesday to meet with FARC representatives, while he himself would meet with political factions including 'no' supporters at home to seek a way forward in the vote's aftermath.

Londono, the Revolutionary Armed Forces of Colombia leader better known as "Timochenko," said FARC remained committed to peace and to using words, not weapons, to build Colombia's future.

Speaking in Havana before his own negotiating team, he said, "to the Colombian people who dream of peace, count on us. Peace will triumph."

On Sunday, however, it was opponents of the peace deal who emerged triumphant, with a razor-thin majority of 50.21 per cent of the vote for 'no' to supporters' 49.78 per cent.

Just 37.4 per cent of of Colombia's nearly 35 million eligible voters went to the polls in the nationwide vote. The turnout was lower than expected, due in part to torrential rains from Hurricane Matthew along the country's Caribbean coast.

Less than a week ago, on Monday, Santos and Londono had stood before thousands in Cartagena to sign the peace accord ending more than a half-century of armed conflict between Colombia and FARC.

But Santos had made ratification of the peace accord by Colombia's people a condition of the deal - a gamble that he lost with the accord's surprise defeat at the polls.

In the weeks leading to the vote, opinion polls had shown a consistent, if narrow, lead for supporters of the accords.

The treaty included provisions for political participation by the FARC and transitional justice for war crimes, as well as rural reforms and reintegration of demobilized guerrillas into civilian life.

Opponents of the deal led by former president Alvaro Uribe had complained it conceded too much to FARC, which funded its violent campaign against state power with drug trafficking, kidnapping and extortion.

The result of the referendum means it is unclear how the Colombian peace process will proceed.

Santos said the bilateral cease-fire agreed in August would remain in effect for now.

Both sides said before the vote they were unwilling to renegotiate the terms. While some analysts said a restart to talks in the case of a "no" vote would be technically possible, others said a return to hostilities would be as well.

The accord was meant to end 52 years of conflict that began in the 1960s over inequality and rural land rights.

Since then, the violence has killed more than 220,000 people and displaced millions.

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