By Robert Campbell

LA JOLLA, California (Reuters) - The United States accused unnamed members of Venezuela's left-wing government on Wednesday of conspiring against neighboring Colombia by supporting Marxist guerrillas.

Assistant Secretary of State Thomas Shannon, the top U.S. diplomat for Latin America, said files discovered on a rebel chief's computer in March contained "troubling" evidence about ties between some Venezuelan officials and guerrillas of the Revolutionary Armed Forces of Colombia, or FARC.

"It will either have to commit itself to using its relationship with the FARC to promote peace or it will have to explain why members of its government are conspiring against a democratic neighbor," Shannon said in a speech in California.

Venezuelan President Hugo Chavez, a fierce critic of U.S. policies, has dismissed claims that his government is providing support to Colombian FARC rebels as part of a smear campaign.

He has clashed repeatedly with Colombian President Alvaro Uribe, a conservative and Washington's closest ally in Latin America, and has called for foreign governments to give more political recognition of the FARC.

U.S. and EU officials label the FARC a terrorist group.

The laptop computer files were found after Colombian forces raided inside Ecuador to kill a top FARC commander at his camp across the border. Colombia says documentation in the laptops shows evidence of rebel ties to both Venezuela and Ecuador.

The raid sparked a regional crisis when briefly raised fears of war when Chavez threatened to send troops to the border with Colombia.

Chavez and Ecuador's President Rafael Correa claim Colombia is using the files as an excuse to attack both countries.

Interpol was called in to investigate whether the files were tampered after they were seized, and is expected to release the results of its probe on Thursday.

Washington has often labeled Chavez as a threat to regional stability. The Venezuelan leader says the White House wants to topple him as he seeks to counter U.S. free trade and foreign policies in Latin America with a call for socialist ideas.

Chavez won a diplomatic victory by persuading FARC rebels to free six of its hostages from jungle camps early this year and he said on Wednesday he would renew efforts to win more releases.

The most prominent hostage is French-Colombian citizen Ingrid Betancourt, a former presidential candidate in Colombia.

French President Nicolas Sarkozy has made her release after years in rebel camps a top foreign policy objective, and he believes Chavez is the best hope to win her freedom.

"I just this minute told Sarkozy that we will keep trying to make contact with the FARC to try and rebuild the road to liberation, to a humanitarian agreement and peace in Colombia," Chavez said on state television on Wednesday.

Uribe has always been wary of Chavez's involvement in hostage talks, and formally removed him from the process last year. The FARC later released several hostages to Chavez as a gesture of good will.

(Additional reporting by Frank Jack Daniel in Caracas; Writing by Patrick Markey; Editing by Kieran Murray)