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 spiraled in the wake of an air strike that killed some two dozen rebel fighters, according to a source close to the government.

The government delegation made no statements as it arrived for Monday's talks.

Hopes for a breakthrough in the peace process had risen in December when the Revolutionary Armed Forces of Colombia 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 ordered the resumption of air strikes.

The military announced it killed 26 rebels Friday in an air strike and ground offensive in Cauca, then 10 more in strikes in the northwestern department of Antioquia over the weekend.

The FARC, with an estimated 8,000 members, suspended their ceasefire in the wake of the first strike, which they say killed 27 of their fighters.

- Government 'took advantage' -

The country's second-largest guerrilla group, the National Liberation Army (ELN) voiced its solidarity with its "sister organization" the FARC.

The ELN, which has an estimated 2,500 fighters, has held preliminary talks with both the government and the FARC on joining the peace process, but has so far not opened formal negotiations.

"The daring decision to declare an indefinite unilateral ceasefire is a bold gesture to create a climate favorable to peace," the ELN said in a statement.

"The Santos government, far from understanding that gesture, took advantage of it to make military gains."

Santos, who narrowly won re-election last year promising to bring the peace talks he started to fruition, has defended Friday's air strike as a "legitimate action."

He called for accelerated negotiations, saying the talks have carried on for a full year "without any substantial advance."

The FARC has repeatedly urged Santos to agree to a bilateral ceasefire, but the president has refused to consider a truce without a final peace deal in place.

The talks in the Cuban capital have so far achieved partial deals on several issues, including political participation for rebels and ending the drug trafficking that has fueled the conflict.

But a final deal remains elusive.

The conflict has killed more than 200,000 people since the FARC was launched in 1964 in the wake of a peasant uprising.

It has also uprooted some five million people, drawing in a web of leftist rebel groups, right-wing paramilitaries and drug traffickers in a half-century of violence.