Government forces in Colombia have killed a FARC brigade commander close to the group's chief negotiator, the country's defence minister said.

Juan Carlos Pinzon, the defence minister, said that Jacobo Arango, a FARC commander in a northwestern area straddling Cordoba and Antioquia provinces, a known drug route, was among six rebels killed in an assault on Thursday.

"It's a strike of great importance," he told reporters on Friday.

Arango was close to chief FARC peace negotiator Ivan Marquez, who was also Arango's direct commander, and he had been a rebel for more than three decades, Pinzon said.

Fighting has intensified since a unilateral FARC ceasefire expired on January 20, with guerrillas taking hostages, killing soldiers and blowing up oil and energy infrastructure.

Government security forces have also stepped up operations. The violence comes even as the two warring sides have been holding peace talks in Havana to try to end a five-decade-long war that has killed tens of thousands of people.

Peace process

Earlier this week, FARC freed three kidnapped oil contractors, but six guerrillas and five government soldiers were killed across the country.

Marquez questioned whether the government was serious about peace - the same doubt Bogota levelled at the guerrillas earlier this week.

"Now there have been many and strident government 'No's' to all our initiatives for peace in Colombia," Marquez told journalists on Friday in Havana.

President Juan Manuel Santos has said he wants to achieve a peace deal within a year, and the FARC, or the Revolutionary Armed Forces of Colombia, have called for a bilateral truce.

However, Santos has rejected the idea of a ceasefire until a deal is signed.

The area where Arango was killed had historically been a region where right-wing paramilitaries fought guerrillas for control and is near the site where paramilitary leaders agreed with the government to demobilise in the early 2000s.