Peace talks to end Colombia's five-decade conflict were plunged into fresh crisis Friday after FARC guerrillas suspended their unilateral ceasefire in response to a government air strike that killed 26 rebels.

The December ceasefire announcement by the Revolutionary Armed Forces of Colombia (FARC) had raised hopes that the two-year-old peace negotiations were approaching a breakthrough. But tensions have spiralled since the rebels killed 11 soldiers in an ambush last month.

On April 15, the day after the ambush, a furious President Juan Manuel Santos ordered the military to resume air strikes against the leftist FARC, which he had suspended on March 11 in recognition of their ceasefire.

Thursday night's air strike and ground attack was the deadliest assault on the FARC since that announcement.

It targeted a FARC base in the western department of Cauca, the FARC stronghold where the rebels carried out the ambush.

The strike came on the same day the government and FARC opened a new round of peace talks, seeking to make progress on ending a conflict that has killed more than 200,000 people.

The FARC, which has defended the April ambush as a "defensive" action taken against an army siege, vehemently condemned the air strike.

The rebel fighters were "murdered in the deep of night and sound asleep, with 250-kilogram (500-pound) bombs, without a chance to fight back," Pastor Alape, a member of the FARC negotiating team, wrote on Twitter, calling it "a treacherous and degrading act."

Soon after, the rebel negotiating team announced on its blog that the FARC was ending its unilateral ceasefire after six months.

"We didn't plan to suspend the ceasefire... but the incoherence of the Santos administration has achieved it, after five months of ground and air offensives against our units across the country," they said.

They said they were still committed to continuing the talks, but reiterated their call for a bilateral ceasefire -- which Santos has repeatedly refused to grant without a final peace deal.

"Against our will we have to pursue dialogue in the midst of confrontation," they said.

"Although Santos has announced he will maintain the offensive, we insist on the need to grant the bilateral ceasefire the national majority has demanded with such insistence, for the health of the peace process and to avoid more victims."

A scheduled meeting Friday between government and rebel negotiators on their landmark agreement to cooperate on clearing the landmines that litter the Colombian countryside was called off, a Cuban foreign ministry official told AFP.

The two sides were expected to go ahead with the next scheduled meeting on Saturday.

Cuba is hosting the peace talks.

- Flare-ups strain talks -

Thursday's attack involved both military and police.

It targeted a FARC unit that the army blames for a November attack on the Pacific island of Gorgona that killed one of its lieutenants, a military source said.

Santos defended the strike, which he called a "very important blow" against the FARC.

"This is a legitimate action by the state in defense and protection of its citizens. These are the rules of the game that we have established."

After the FARC announced the suspension of their ceasefire, he vowed the military was "prepared" for any new attacks.

But in a Twitter post, he called on the FARC to double down on the peace process.

"Gentlemen of the FARC: It is time to accelerate negotiations. How many more deaths are needed to understand that now is the time for peace!"

The peace talks began in 2012 and have been continually derailed by attacks from both sides, including the FARC's capture of an army general in November.

The negotiations have reached partial deals on several issues, including political participation for FARC members and fighting the illegal drug trafficking that has fueled the conflict.

But a final accord remains elusive.

The FARC was founded in 1964 and has about 8,000 fighters.

FARC negotiators recently met the National Liberation Army (ELN), another leftist rebel group with about 2,500 fighters, on joining the peace process.

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