President Juan Manuel Santos demanded that Colombia's largest rebel group immediately release an army general it captured, saying the resumption of suspended talks to end the half-century-old conflict depend on it.

Santos addressed the nation Monday night little more than 24 hours after Gen. Ruben Dario Alzate, dressed in civilian clothes, was snatched by gunmen along with two others while visiting a hamlet along a remote river in western Colombia. A soldier who managed to flee in the group's boat said the rebels belonged to the recalcitrant 34th Front of the Revolutionary Armed Forces of Colombia, or FARC.

It's the first time that the guerrillas have taken an army general captive and couldn't have come at a worse moment for Santos.

Even before Santos suspended two-year-old peace talks, frustration with the slow progress and the guerrillas' refusal to wind down attacks had been building. Earlier this month, the FARC captured two soldiers during intense fighting in northeast Colombia and killed two Indians who confronted rebels hanging up revolutionary banners on their reservation. It has since offered to free the soldiers.

Calling Alzate's abduction "totally unacceptable," Santos ordered government peace negotiators not to travel Monday to Cuba as planned for the next round of peace talks until Alzate and the two others — an army captain and a female lawyer advising the army on a rural energy project — are freed.

"The FARC have to understand that, although we're negotiating in the middle of the conflict, peace doesn't come by resorting to violence and undermining confidence," Santos said.

Amid the tough talk, little is known about the general's whereabouts or why he apparently violated military protocol and set off on the Atrato River in the dangerous zone dressed as a civilian without bodyguards. A massive search operation mounted Monday has so far yielded few leads and residents of the 800-person hamlet of wooden shacks where the group was taken told local media they didn't know about the visit.

The FARC considers captured military personnel to be prisoners of war even though it freed all soldiers in its control and swore off the kidnapping of civilians on the eve of talks in 2012.

It also has been clamoring for a cease-fire while peace talks continue, something Santos has rejected for fears it would allow the guerrillas to regroup like they did in the last attempt at peace that ended in 2002.

The FARC's 34th Front is among the group's most entrenched and fiercest fighting units, based in the dense, water-logged jungles around Quibdo. Its members repeatedly violated unilateral cease-fires declared by the FARC leadership in Havana during elections and Christmas holidays.

A spokesman for the FARC in Havana declined to comment, saying the rebels' negotiators were still investigating the incident and would comment at a press conference Tuesday.

Santos, who was re-elected in June, has staked his presidency on reaching a deal with the FARC. But he's struggled to overcome doubts from conservative opponents and members of the military who fear he's ceding too much power to the rebels.

"While the FARC talks about peace in Havana, here they're committing all sorts of atrocities," said former Peace Commissioner Camilo Gomez, adding that the talks could unravel without a major overhaul to demand the FARC demonstrate its commitment to peace. "This could be the straw that breaks the camel's back."

Analysts say that the capture appears to have been accidental. Securing the general's release could be difficult: after a decade of heavy losses inflicted by the U.S.-backed military, the FARC leadership's operational command over its estimated 8,000 troops has been greatly reduced and just getting messages to the front lines could take several days.

Alzate, 55, is among Colombia's most-decorated soldiers. A graduate of the U.S. Army War College and Command and General Staff College in Kansas, he previously oversaw the military's anti-kidnapping unit. In January, Santos named him commander of the newly established Titan Task Force, a 2,500-man counterinsurgency force operating from Quibdo.

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Associated Press writers Camilo Hernandez in Bogota and Andrea Rodriguez in Havana contributed to this report.

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Follow Joshua Goodman on Twitter: @APjoshgoodman