Colombia's president has rejected the involvement of US civil rights activist Jesse Jackson in mediating the release of a former US Marine kidnapped by FARC rebels.

President Juan Manuel Santos said on Saturday he would not allow the FARC rebels a "media spectacle" by using Jackson as a negotiator for the release of kidnapped Afghanistan war veteran Kevin Scott Sutay.

FARC, or Revolutionary Armed Forces of Colombia, kidnapped Sutay in June as he trekked through jungle in southeastern Colombia despite warnings from the police to abandon the trip through what it said was a "red zone" of guerrilla activity.

The FARC had requested that Jackson help with the freeing of Sutay on Saturday. They said they were holding the former Marine as a prisoner of war and accused him of being a mercenary.

Jackson said from Cuba, where he is on a visit to try to improve ties between the communist-run island and the US, that he agreed to help and would aim to arrive in the South American country within a week.

"We have made contact with the (US) State Department urging them to contact the next of kin of Kevin Scott and let them know his release is imminent,'' Jackson said.

However, hopes of an imminent release dimmed by the evening whenSantos rejected Jackson's intervention.

Release setbacks

"Only the Red Cross will be authorised to facilitate the handover of the North American kidnapped by the FARC," he said on Twitter.

"We will not allow a media spectacle."

The FARC appeared ready to release Sutay in July until Santos rejected its initial request that a leftist politician, Piedad Cordoba, oversee the release.

It then began accusing Sutay of being a mercenary and made no further offer to free him until its request to Jackson.

Jackson appealed to the FARC to release Sutay when he was in Colombia 10 days ago to attend an international conference of Afro-descendent mayors and government officials, saying it would boost its peace talks with the government.

The two sides have been in negotiations hosted by Cuba since last November that aim to end a five-decade conflict that has killed more than 200,000 people. Jackson met a FARC delegation in Cuba during his visit.