Leftist rebels engaged in peace talks with the Colombian government urged the army on Friday to refrain from any attempt to rescue a US hostage they are holding.

Revolutionary Armed Forces of Colombia (FARC) guerrillas claimed that army operations have increased in Guaviare, a central-eastern region bordering Brazil, in an attempt to rescue American Kevin Scott Sutay by force.

Any military rescue attempt would be "inhumane and irresponsible, and put at risk the life of the prisoner," the FARC said in a statement posted on their website www.farc-ep.co.

"We call on the government to be sensible, to avoid an unfortunate outcome such as what has already happened on previous occasions," the group said.

Sutay, a US military veteran who served in Iraq and Afghanistan, was captured on June 20 in Guaviare two days after arriving in Colombia as a tourist.

The US ambassador has said he is a private citizen and has nothing to do with the US military mission in Colombia, a close US ally.

The FARC however said his capture was evidence of "the active participation on the ground of American military and mercenaries in counter-insurgency operations in which they appear under the euphemism of contractors."

The FARC has said it was prepared to release Sutay to a group of intermediaries who would include representatives of the International Committee of the Red Cross and Piedad Cordoba, a leftist former senator who has previously served as a go-between.

But President Juan Manuel Santos has said he would not allow Cordoba to be part of the commission, only ICRC representatives.

The FARC statement comes as US Secretary of State John Kerry is set to visit Colombia on Monday.

Peace talks between the rebels and the government opened last November in Cuba, the fourth attempt since the 1980s to end Latin America's longest-running armed conflict.

In early 2012 the FARC committed unilaterally to stop kidnapping civilians.