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At least 20 Farc rebels have been killed in Colombia after the military launched bombing strikes on one of their camps, the army says.

Saturday's raid is said to be the biggest military operation against Farc since peace talks began in October.

The camps were in Narino province near the Ecuadorian border, commander Gen Leonardo Barrero told AFP news agency.

The strike comes as President Juan Manuel Santos said the rebels had less than a year to abandon their weapons.

In November, the Farc announced a ceasefire set to last until 20 January.

Mr Santos, however, has rejected calls for a government-led truce until a final agreement has been reached.

Speaking on Sunday, he said his administration would offer all necessary guarantees so the Farc could disarm and join the political process as a legal party.

"This has to be a process of months, rather than years," the president said.

Mr Santos said any attempts to delay disarming were unacceptable. The Farc has not responded to Mr Santos's deadline.

The president made his comments after the rebel group acknowledged that it was holding what it called "prisoners of war", contradicting its previous denials.

In February, the rebels announced that they would stop all political abductions and kidnappings for ransom.

But victims' groups say the kidnappings have continued and not all hostages have been released.

The peace talks, aimed at ending five decades of conflict, began in Norway on 18 October before moving to Cuba's capital, Havana, a month later.

Negotiations focus on land reform, the end of armed conflict; guarantees for the exercise of political opposition and citizen participation, drug trafficking; and the rights of the victims of the conflict.

The first stage of the dialogue ended last Thursday, with talks due to resume on Wednesday.

All previous attempts to reach a deal have failed.

The government ended the last peace talks in 2002, accusing the rebels of trying to regroup in a demilitarised zone.

Hundreds of thousands of people have died since the conflict began in Colombia, with millions more displaced.

The Farc numbered 16,000 in 2001 but are now thought to have some 8,000 fighters.