HAVANA—Colombia's main rebel group announced a unilateral cease-fire on Monday as it began much-anticipated peace talks, but the Bogota government responded that it would continue military operations.

Top negotiator Ivan Marquez said the Revolutionary Armed Forces of Colombia would halt all acts of sabotage and attacks against government and private property starting at midnight Monday and running through Jan. 20.

He made the announcement as negotiators for both sides entered the talks in Havana without other comment.

Marquez said the move was "aimed at strengthening the climate of understanding necessary for the parties to start a dialogue."

Hours later, Defense Minister Juan Carlos Pinzon told reporters in the Colombian capital that while the government hoped the FARC would keep its promise, "history shows that this terrorist organization has never kept its word. It's very difficult to believe."

He added that Colombian security forces have "the constitutional duty to pursue all criminals who have violated the Constitution."

"As a result, the terrorists of the FARC are being pursued for all the crimes they have committed over so many years and not for future crimes," said Pinzon, one of President Juan Manuel Santos' most trusted collaborators.

The FARC had sought a cease-fire before entering the talks. But Santos firmly rejected halting military operations, intent on obtaining tangible results in the negotiations from an insurgency that has been weakened militarily in recent years.

Piedad Cordoba, a leftist former Colombian senator who has served as a go-between with the Western Hemisphere's last remaining major insurgency, said the unilateral cease-fire gives the FARC "credibility and legitimacy."

"It certainly puts political pressure on the Santos government" not to attack the rebels during the talks, said Adam Isacson, an analyst at the Washington Office on Latin America.

The talks got under way under threatening skies at a convention center in Havana, with most rebel and government negotiators dressed casually in short-sleeve shirts or guayaberas.

The rebel delegation came sporting a life-size cardboard cutout of Simon Trinidad, a FARC leader symbolically named to the negotiating committee. He is serving a 60-year jail term in the United States for hostage-taking conspiracy and the rebels consider him a prisoner of war.

Among the rebels at the talks was Tanja Nijmeijer, a 34-year-old polyglot Dutch woman who joined the rebels a decade ago and was a last-minute addition to the rebel delegation. Nijmeijer, the only woman at the negotiating table, entered the convention center wearing a dark beret with a FARC insignia.

Cuba is playing host to the talks in Havana following a formal inauguration in Oslo, Norway last month. The FARC has been at war with successive Colombian government half a century and the drug trade-fueled conflict has claimed tens of thousands of lives, mostly civilians, and included killings of suspected rebel sympathizers by right-wing death squads allied with the military.

There is no deadline for agreement, though Santos has said he expects results in months, not years, or he will halt the negotations.

The talks, the result of seven months of secret negotiations in Havana, follow four failed efforts since the early 1980s.

A cease-fire reach with the FARC in the first of those efforts fell apart amid accusations of mutual failures to honor it.

Universidad de los Andes political scientist Sandra Borda said in Bogota that the rebels' cease-fire announcement has her wondering whether the FARC's leadership can pull it off.

"If it works, it will substantially change the tone of these negotiations," said Borda, and become "a mechanism of pressure on the government to stay at the table."

Land reform, the heart of the conflict, is at the top of the agenda and the negotiators vowed in a roadmap agreement signed Aug. 26 that both sides would keep the negotiations secret pending joint public progress reports.

The Colombian government is hoping peace leads to greater foreign investment, including the extractive industries, and says its economic model is not negotiable. But the FARC is insisting on nationalizing natural resources.

The government has promised to return millions of acres of stolen land to displaced peasants, one of the rebels' main demands, and compensate victims.

The 9,000-strong FARC is being asked as a condition of peace to help end the cocaine trade that has funded its struggle.

Colombians also want it to account for the dozens of kidnap victims who have disappeared in its custody and other noncombatants it is accused of killing.

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Paul Haven in Havana, Cesar Garcia and Vivian Sequera in Bogota, Colombia, and Frank Bajak in Lima, Peru, contributed to this report.