BOGOTA, Colombia: Eleven soldiers were killed and 19 injured in an attack by leftist guerrillas in Colombia, likely a major violation of the rebels' pledge of a unilateral cease-fire that throws into doubt the future of peace talks.

The attack occurred around midnight on Tuesday in the volatile southwest department of Cauca when an army unit on a routine patrol was surprised by guerrillas firing homemade explosives and grenades. A corporal and 10 others were killed during the ambush by a unit of the Revolutionary Armed Forces of Colombia, or FARC, the army's third division said in a statement.

President Juan Manuel Santos offered his condolences to the victims' relatives and traveled on Wednesday to the combat zone in a rural output called La Esperanza to investigate the circumstances of the attack.

"This is precisely why we want to end the war," he said in a message posted on Twitter.

Colombians immediately took to social media to denounce the attack and call on Santos to abandon the peace talks, which have been making steady progress toward the goal of ending a half-century of fighting.

"Santos, don't betray us anymore," former President Alvaro Uribe, a fierce critic of the peace process, said on Twitter. "Don't justify the murder of our soldiers with this talk of war you want to end."

FARC negotiators in Cuba committed in December to a unilateral cease-fire to promote peace talks that have taken place on the communist-led island over the past two years, saying they would only fire weapons if attacked by the armed forces.

While adherence to the leadership's command has been less than complete, the latest attack is the most serious breach to date and could destroy momentum toward a deal that has been building since the FARC last November shocked much of Colombia by releasing an army general that it had accidentally captured.

Last month, both sides agreed on a plan to begin jointly removing dangerous land mines that litter large parts of the countryside. Soon afterward the government suspended all aerial bombings of guerrilla camps, an order that Santos extended this month.

Such good-faith gestures are seen as crucial in rallying support for a deal when some of the thorniest issues, such as whether rebel leaders will serve time in jail for atrocities and the fate of US drug indictments against the FARC's leadership, remain to be negotiated. The two sides have already reached deals on land reform and political participation for former rebels.

The latest attack underscores one of the biggest obstacles in the way of a deal: the FARC leadership's lack of control over the estimated 7,000 troops still on the battlefield. That's especially true in turbulent, lawless areas like Cauca, where rebel commanders are known to be heavily involved in drug-trafficking.

From Havana, the guerrilla commander known by his nom de guerre Pastor Alape said he was unaware of the circumstances that led to Tuesday's combat. But he blamed the incident on Santos' refusal to declare a bilateral cease-fire, something the FARC has been urging since the start of negotiation.

"It's urgent for the nation, something the entire country is waiting for," he said.