The Colombian government and FARC guerrillas resumed peace talks Monday in Havana amid heightened tensions following air strikes that killed dozens of rebels.

Rebel leader Pablo Catatumbo condemned the government offensive as he arrived for the talks, which opened in November 2012 but have made only halting progress on ending the five-decade-old conflict.

"Without a doubt, the tragic events are a step backward in what we've achieved up to now at the negotiating table," he said.

"They can't expect military pressure or threats to break our will to fight. That's the wrong path and it's obvious that peace will never be reached by escalating the conflict."

The two sides had postponed the talks on Friday "by common agreement" as tensions spiralled in the wake of an air strike that killed 26 FARC fighters, according to a source close to the government.

Hopes for a breakthrough in the talks had been raised in December when the FARC announced an indefinite unilateral ceasefire.

President Juan Manuel Santos partially reciprocated in March by suspending air strikes on FARC positions.

But the outlook has deteriorated since the FARC killed 11 soldiers last month in an ambush in the western department of Cauca, a rebel stronghold.

They defended the attack as a "defensive" action taken against an army siege, but a furious Santos called off the suspension of air strikes.

The military killed 26 rebels last week in an air strike and ground offensive in Cauca, then killed eight more in strikes in the northwestern department of Antioquia at the weekend.

The FARC suspended their ceasefire in the wake of the first strike.

The conflict has killed more than 200,000 people since the Revolutionary Armed Forces of Colombia (FARC) was launched in 1964.