* Santos says only Red Cross can help in ex-Marine's release
* President insists will deny FARC a liberation "media show"
* Jackson asked FARC to release hostage, offered to assist
Colombia's president rejected the involvement of U.S. civil rights activist Jesse Jackson in overseeing the release of a former U.S. Marine kidnapped by FARC rebels, saying on Saturday he would not allow the guerrillas a "media spectacle."
The FARC kidnapped Afghanistan war veteran Kevin Scott 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, or Revolutionary Armed Forces of Colombia, had requested on Saturday that Jackson assist with the freeing of Sutay. They say they are holding the former Marine as a prisoner of war and accuse him of being a mercenary.
Before reaching Colombia, Sutay is thought to have been backpacking through several Central and South American countries.
Jackson told reporters in Cuba, where he is on a visit to try to improve ties between the communist-run island and the United States, that he agreed to help and would aim to arrive in Colombia within a week.
Hopes of an imminent release dimmed by the evening when President Juan Manuel Santos rejected Jackson's intervention via Twitter, reiterating he would deny the FARC an ostentatious liberation it has called a "humanitarian" gesture.
"Only the Red Cross will be authorized to facilitate the handover of the North American kidnapped by the FARC," the tweet read. "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.
The FARC began as a communist-inspired peasant army fighting to reduce inequality and redistribute land. Its numbers have been halved in the last decade by a U.S.-backed offensive.
The rebels said in February 2012 they would stop taking hostages to raise money for their armed struggle, but reserved the right to continue taking prisoners of war.