The military made preparations to retake a rebel safe haven after Colombia's president said the rebels had broken off peace talks and he gave them 48 hours to vacate the Switzerland-sized zone.

Some have feared a failure of the peace talks would lead to a bloodier phase in Colombia's 38-year civil war. But a grave Andres Pastrana said now is the time "for a cool head and lots of calm."

"We can't fall into exaggerated alarmism or unnecessary panic," he said during a national address. "We won't enter into a terrible, merciless war."

The rebels had no immediate comment. Hours before Pastrana's announcement, rebel spokesman Raul Reyes had said that they hoped talks would continue until at least Jan. 20, when the safe haven was set to expire.

Before Pastrana had even made the announcement, the army began moving troops. More than a dozen tanks and 10 trucks carrying soldiers were seen on the streets of Bogota on Wednesday evening, heading to a base in the south of the city, closer to the rebel safe haven.

Troops throughout the country were put on high alert, army spokesman Maj. Jose Espejo said.

Pastrana blamed the collapse of the talks on the intransigence of the Revolutionary Armed Forces of Colombia, or FARC.

"Today I have to tell Colombians, with regret, but above all with realism and responsibility, that the FARC keeps placing obstacles in front of the peace process, making it impossible for us to keep advancing with the process," Pastrana said.

"The FARC has 48 hours, as agreed, to retire from the zone," he said, referring to the original timeframe for them to abandon the safe haven if talks failed.

Though he didn't specify what could resurrect the peace talks, Pastrana said "this is not final."

"I will keep the doors to dialogue and negotiation open because I remain convinced that this is the best way out of the internal conflict that our country is suffering from," he added.

Pastrana ceded the safe haven to the FARC three years ago to jump start the talks. The zone in southern Colombia has been the site of the sporadic negotiations, and the president has renewed it several times.

But Pastrana said the rebels had refused to discuss important issues like a cease-fire.

The presidential peace negotiator, Camilo Gomez, said earlier in the day that the FARC had withdrawn from the process, but the rebel spokesman said he was lying.

"He lied to the country and the international community when he said the FARC had asked for 48 hours ... for the armed forces to enter the zone after not coming to an agreement," Reyes said.

Gomez spent the past week in the safe haven trying to revive the talks, which were suspended by the rebels last October after the military increased patrols along the borders of the zone.

Before returning to Bogota late Wednesday, Gomez said the government would guarantee the safety of civilians within the safe haven.

A municipal human rights official in the town of Meseta, inside the rebel zone, said the possibility the zone would be canceled had people "scared."

"We've been doing well, despite the neglect of the government," said Francisco Arteaga. "All the residents are very scared now."

The only airline with commercial flights into the zone, Satena, suspended those flights Wednesday afternoon.

Roughly 3,500 people are killed every year in Colombia's civil war, which pits the FARC and a smaller rebel group against government forces and an illegal right-wing paramilitary group.

Washington supports the Colombian military with a billion-dollar aid package aimed at improving the government's ability to combat drug trafficking.

Though the aid is restricted to counternarcotics efforts, the rebels, who finance their fight by taxing the drug trade, see it as thinly veiled intervention in Colombia's conflict.