Colombia's leftist FARC guerrillas said Monday they will maintain their unilateral ceasefire despite a recent clash with the army, except if their fighters come under siege.

The two-year-old peace process between the Revolutionary Armed Forces of Colombia (FARC) and the government suffered a setback last week when the rebels killed 11 soldiers in what the army called an unprovoked attack.

After the incident, President Juan Manuel Santos ordered the military to resume air strikes on the guerrillas, which he had suspended in March in recognition of their ceasefire.

The FARC's chief negotiator at the talks, Ivan Marquez, said the rebels were not calling off the ceasefire they declared in December.

"We are maintaining the indefinite, unilateral ceasefire, as long as we are not subjected to a permanent siege by troops," he told journalists in Havana, where the peace talks are being held.

He called for both sides to continue negotiations to end the five-decade conflict.

Last week's clash in the western rebel bastion of Cauca also left two FARC fighters dead and some 20 soldiers wounded, according to the army.

The attorney general said the FARC attacked in a "late-night ambush" while soldiers were sleeping.

But the FARC insist they only retaliated because their forces were under siege. They say they did not break their ceasefire pledge, in which they vowed only to act in self-defense.

"You have to understand that sometimes offensive actions are deployed in the course of legitimate self-defense," Marquez said.

The FARC want the government to commit to a bilateral ceasefire, but Santos has repeatedly refused to do so without a final peace deal.

The Colombian conflict has killed more than 200,000 people and uprooted five million since the Revolutionary Armed Forces of Colombia was founded in 1964.