BOGOTA (Reuters) - Colombia's peace negotiations with leftist FARC rebels have hit a critical low as the group steps up violent attacks and the government could walk away from the process unless the group shows more commitment, the government's top negotiator said.

The government has been in Cuba-hosted talks with the Revolutionary Armed Forces of Colombia or FARC since late 2012 to end a 50-year war. Deals have been reached on most of the agenda, but the process is now under unprecedented strain.

"The peace process is at its worst moment since we began talks ... I want to tell the FARC in all seriousness, this could end. Some day, it's probable that they won't find us around the table in Havana," Humberto De la Calle told local journalist and writer Juan Gossain, in an interview distributed to the press.

The FARC unleashed a wave of bomb attacks on oil pipelines in recent weeks, rupturing them close to rivers and causing an environmental disaster that is expected to take two decades to clean up and has already reached the Pacific coastline.

Optimism that had grown over the peace talks, boosted by the FARC's agreement to clear landmines, was shattered after the FARC ambushed and killed 11 soldiers in the southwest in April. That led the government to resume air raids of FARC jungle bases in turn prompting the rebels to abandon a unilateral ceasefire.

The FARC's insistence the government agree to a bilateral ceasefire even as its negotiators flatly reject the idea some members should pay jail time for the group's worst crimes, have also driven a rift between them and dimmed prospects for a deal.

De la Calle said the government was prepared to consider a bilateral ceasefire before a deal is signed, if the FARC accepts judicial responsibility for the violence it has perpetrated and only if it also abstains from extortion and the drug trade.

In a conciliatory move on Friday, the FARC leadership in Havana said they were seeking to "de-escalate" the conflict after weeks of attacks in which several army troops and police were also killed. The group's numbers are falling and all-out war is no longer seen as a realistic option.

Even if the peace process survives, its biggest obstacle may prove a skeptical Colombian public which will approve or reject any peace deal in a referendum. Its support has faded as talks drag on.

"The harsh truth is that Colombians don't believe in the peace process," De la Calle said.

(Editing by Michael Perry)