By Jack Kimball and Luis Jaime Acosta
BOGOTA, Feb 1 (Reuters) - Colombian government forces have killed a FARC brigade commander close to the Marxist group's chief peace negotiator, the defense minister said on Friday, as combat heats up after the expiration of a unilateral guerrilla ceasefire.
Defense Minister Juan Carlos Pinzon 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.
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 Jan. 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 while the two warring sides talk peace in Havana to try to end a five-decade-long war that has killed tens of thousands of people.
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 leveled 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.
"The Colombian people have heard through the media the repeating of the government order to intensify the war, now strangely it's accompanied by complaints about its consequences, while they call us cynics," he added.
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 demobilize in the early 2000s.
Today, the region is a microcosm of the security challenges plaguing many of the areas of Colombia where guerrillas and powerful gangs linked to the former paramilitaries fight for control over drugs and territory.
A U.S.-backed military offensive against rebels and drug gangs since 2002 has made vast strides in improving security in what is Latin America's No. 4 oil producer, opening up swathes of the country to investment, especially in the oil and mining sectors. But Colombia has yet to fully escape its bloody past.
Colombian forces killed 353 members from all the country's rebel groups in the January to October period last year, almost as many as killed in the full-year 2012, but down from highs of around 2,000 in the early 2000s, defense ministry data showed.
The ministry does not break down security forces killed by insurgents alone, but 336 members were killed while on duty in the January to October period in 2012, down 94 percent from the same period in 2011, the data showed. (Additional Reporting by Nelson Acosta in Havana; Editing by David Brunnstrom)