BOGOTA, Colombia (AP) — Colombia's government has rebuffed a unilateral truce declared by the country's largest rebel group, saying conditions demanded by the guerrillas' are unacceptable until a peace deal is reached.

The Revolutionary Armed Forces of Colombia on Wednesday said they would lay down their weapons for an unlimited period to bolster peace talks that have been held in Cuba for the past two years.

But Latin America's oldest and strongest insurgency also said it would call off the cease-fire if their units were attacked by Colombia's U.S.-backed military — a condition that appears to have doomed the gesture given the government's longstanding refusal to enter a bilateral truce out of fear it would give the rebels an opportunity to re-arm.

Those concerns were again apparent Thursday when President Juan Manuel Santos said he couldn't accept the rebels' demand the truce be verified by several Latin American nations and the international Red Cross. Such outside verification would have to wait until a deal to end a half century of hostilities is reached, he said in a statement.

Santos, a defense minister in the hawkish government of former President Alvaro Uribe, also said he would continue to fulfill his constitutional duty to protect and guarantee the safety of all Colombians.

Nevertheless, Santos said he "values" the rebels' gesture as a way to begin de-escalating a conflict that still claims hundreds of civilian lives every year and is fueled by the smuggling of cocaine and other criminal activity.

It's unclear where the government's response leaves the cease-fire, which is set to take effect midnight Saturday.

Although the FARC have declared temporary cease-fires before, around Christmas and elections, this would be the first time they've offered to indefinitely lay down their weapons nationwide since the 1980s.

The gesture comes as both sides struggle to recommit themselves to talks that were almost derailed following the capture last month of an army general — the highest-ranking officer ever held by the FARC.

Santos briefly suspended negotiations but the crisis was quickly overcome after the FARC freed the general two weeks later.

The two sides already have reached agreements on agrarian reform, political participation for the FARC and how to jointly combat illicit drugs in what was long the world's largest cocaine producer.

But some of the thorniest issues remain unresolved, including how the FARC would lay down their arms and whether commanders would face prosecution for atrocities and drug-trafficking.

Those concerns prompted thousands of Colombians, led by the still-powerful Uribe, to march over the weekend in major cities to demand that rebel leaders not be granted an amnesty that would allow them to escape justice for mass killings, kidnappings and drug trafficking.

Uribe, whose conservative government launched the military offensive credited with pushing the FARC deeper into the jungles, on Wednesday called the guerrillas' conditioning of its cease-fire on the government's withholding of its own firepower a form of "blackmail."