UPDATE 6-Colombian rebels call truce as peace talks start

* Ceasefire starts at midnight, to run through Jan. 20

* Three previous peace processes failed

* Government wants agreement in matter of months

HAVANA/BOGOTA, Nov 19 (Reuters) - Colombia's Marxist rebels

called a two-month unilateral ceasefire on Monday, the first

truce in more than a decade, as delicate peace talks began in

Cuba to try to end a half century of war.

Colombian President Juan Manuel Santos' government

reiterated, however, that there would be no halt to military

operations until a final peace deal is signed with the

Revolutionary Armed Forces of Colombia, known as FARC.

The rebel group said it would halt all offensive military

operations and acts of sabotage against infrastructure beginning

at midnight on Monday and running through Jan. 20.

"This decision by the FARC is a decisive contribution to

strengthen the climate of understanding needed so the parties

... can achieve the purpose desired by all Colombians," lead

rebel negotiator Ivan Marquez said, standing outside a

convention center for the start of talks in Havana.

The gesture is a sign that the rebels may be keen to push

talks to a successful end - something that was thrown into doubt

by long, drawn-out speeches by its leadership calling for major

changes to Colombia's political system.

The warring sides arrived at the talks in black luxury cars

and will meet almost daily until negotiations end.

A crush of journalists surrounded the bearded, bespectacled

Marquez who stood with other FARC delegates, including Dutch

national Tanja Nijmeijer in Havana's plushest neighborhood.

Some FARC members wore caps and T-shirts of Simon Trinidad,

an official guerrilla negotiator who is in prison in the United

States. Others shouted "Long Live the Army of the People."

The head of the Colombian government delegation, Humberto de

la Calle, smiled and waved as he entered but made no comment.

Speaking from Bogota, Colombian Defense Minister Juan Carlos

Pinzon doubted the sincerity of the FARC's ceasefire pledge.

"Security forces have the constitutional duty to pursue all

criminals that have violated the constitution," he said.

"Hopefully they keep their promise, but history shows that this

terrorist group never complies with anything."

Colombia's war has dragged on for 50 years, taking thousands

of lives, displacing millions more and causing damage to

infrastructure in Latin America's longest running insurgency.

A failure of the latest peace process would mean years of

more fighting and further blight on the reputation of a country

eager for foreign investment and regional clout, yet which has

been unable to resolve its most serious domestic problem.

Residents in western Cauca province, one of Colombia's most

war-ravaged areas, celebrated the FARC ceasefire.

"We hope it's not just two months, we hope that it's

definitive," Orlando Ramos, a resident in Miranda, Cauca, said

on local television.

'GRAIN OF SALT'

The announcement by the FARC could be a breather for oil and

mining companies, the target of many FARC attacks in recent

months as the group sought to hobble Santos' main source of

international revenue.

The war costs Latin America's fourth-largest economy 1 to 2

percentage points of gross domestic product every year,

according to the government, and makes large tracts of arable

land unsafe due to combat or landmines.

"A peace agreement with the FARC could entice more sectors

and investors into Colombia," said Eurasia Group's Latin America

analyst Heather Berkman.

"The opportunities for agriculture production in particular

could reshape the country's export sector, particularly as both

small-scale and larger farmers could produce on land long

off-limits due to security troubles."

Santos wants an agreement within nine months, while the

rebels say the process will likely take longer. The two sides

face plenty of thorny issues in their five-point agenda, which

will begin with rural development.

Previous peace attempts have failed, but both the government

and the FARC have expressed optimism that this time might be

different.

Not everyone is so upbeat though.

"You have to take this announcement with a grain of salt,"

Felix Lafaurie, head of Colombia's National Federation of Cattle

Ranchers, said on Colombian radio.

"I hope this is going to be a sign of the FARC's good will

and not that they'll then take swipes on substantive issues."

The vast majority of Colombians support the peace process,

although they think it will ultimately fail. Even so, the talks

are the biggest gamble in Santos' political career and their

success or failure may decide the outcome of the next election

in 2014.

The conflict dates back to 1964 when the FARC emerged as a

communist agrarian movement intent on overturning Colombia's

long history of social inequality. During the 1990s, the FARC

controlled large parts of the country.

In the early 2000s, billions of dollars in U.S. aid,

improved intelligence and increased mobility began to turn the

tide of the war in favor of the government.

The FARC has lost at least half a dozen top commanders and

been pushed back into remote jungle hideouts in recent years,

though the rebels are far from a spent force and still wage

attacks on security forces and economic infrastructure.

Violence was among the reasons previous peace talks failed.

In the last attempt from 1999 to 2002, the government broke off

negotiations after the FARC hijacked an airplane.

"The FARC have heard the voice of many Colombians, that

rightly have been skeptical about its willingness to reach an

end to the war, given the past," said Juan Fernando Cristo, a

senator for the Liberal Party.

"The decision for a unilateral truce should fill us with

optimism about what's coming at the negotiating table."

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