BOGOTA, Oct 22 (Reuters) - A grenade attack on a local Colombian party campaign headquarters killed two and wounded at least six others on Monday in the latest violence authorities blamed on guerrillas before next week's municipal elections.

Two men lobbed the explosive into the campaign office of a candidate for mayor in Puerto Asis in Putumayo province near the Ecuadorean border. The candidate's sister was one of those killed in the attack by guerrillas, police said.

Colombia's 4-decade-old conflict has eased under President Alvaro Uribe, popular for his U.S.-backed campaign to improve security and fight drug trafficking that has helped fuel violence involving guerrillas and illegal paramilitaries.

But killings, threats and attacks have surged in the buildup to the vote on Sunday for governors, mayors and councils.

Uribe is under pressure to illustrate progress in his reforms as the U.S. Congress threatens to block a free-trade deal due mainly to concerns about political violence by outlawed paramilitaries, especially against labor leaders.

The president says 18 candidates have died, while police say 27 have been killed this year, compared with 35 in elections in 2003. An electoral watchdog says 25 candidates have been murdered this time, up from 15 in the previous vote.

Inconsistencies may be due to ways of counting those who were killed before they formally registered their campaigns.

Authorities blame the FARC rebels -- the Revolutionary Armed Forces of Colombia -- for most attacks as guerrillas seek to show influence. Established as a Marxist-inspired group in the 1960s, the FARC is now involved in cocaine trafficking.

Uribe has negotiated the disarming of paramilitaries who once fought guerrillas. But thousands of former militiamen have joined criminal gangs tied to traffickers and also threaten local officials for control of public contracts, analysts say. (Additional reporting by Luis Jaime Acosta)

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