A peace accord with Marxist FARC rebels would not likely end all bloodshed in Colombia because breakaway renegade fighters and other drug-funded crime gangs would continue to battle government troops, an influential think-tank said on Tuesday.

Half-way through his four-year term, President Juan Manuel Santos is taking the biggest political gamble of his career, negotiating with leftist rebels to try to end a nearly five-decade war that has killed tens of thousands of people.

The talks, which were announced after two years of secret negotiations with the Revolutionary Armed Forces of Colombia, known as the FARC, are the first attempt in a decade to end the war and will start in Norway next month then move to Cuba.

"Fears over peace talks are tactically exaggerated by their opponents. But those promoting a political settlement also need to keep expectations in check. A deal would not eliminate violence," the Brussels-based International Crisis Group said.

"It likely would fail to convince some FARC elements to lay down arms, notably those deeply involved in the drugs trade. There would still be significant security threats from illegal armed groups rooted in the officially demobilized paramilitaries and from other organized criminal gangs."

After pushing through major reforms such as giving back land to displaced peasants, which were seen as paving the way for talks, Santos is trying to do what no other government since 1964 has been able to do - reach a deal with the FARC.

The last attempt at peace more than a decade ago collapsed and was seen as having helped the FARC build up their fighting forces. But 10 years later the rebels are at the weakest in their history after heavy blows to their leadership and funding.

In a 32-page report, the International Crisis Group said that Colombia probably had a "more than even chance" of succeeding in negotiations this time, as the government faces fewer obstacles and a majority of Colombians support the talks.

Santos promises his government has learned from the mistakes of previous leaders who tried but failed to clinch a lasting deal, and he has been cautious when talking about the possible success of the discussions.

POLITICAL FUTURE

A Colombian intelligence source told Reuters that some FARC commanders in southern Colombia, where soil conditions are perfect for cultivation of coca - the raw material for cocaine - are against the peace talks.

Santos' government and the FARC have set an agenda for talks that includes the rights of victims, land ownership in rural areas and cocaine production and smuggling.

The success of peace talks will also determine whether Santos, who was a defense minister in former President Alvaro Uribe's government, stands for re-election in 2014 and whether his ruling coalition will stay together.

The stakes are high for both sides.

"Engaging in peace talks with FARC is a bold gamble. Failure would significantly damage the political capital of the Santos administration and likely pave the way for hardliners to return to power in the 2014 elections, closing the window for a negotiated settlement for a long time," the group said.

"For FARC, failure would likely have grave consequences, especially if it bore most of the blame. Having lost a possibly last chance to end the conflict with a political deal and recognition of its struggle, it could anticipate a future of complete political irrelevance, further heavy military pressure and increasing internal tensions."