President Juan Manuel Santos Tuesday rejected the involvement of a leftist mediator in the handover of a former US Marine held hostage by leftist guerrillas, potentially complicating his release.
Santos said he would only allow the International Committee of the Red Cross to facilitate the release of the American, which would require government safe conduct guarantees.
Santos accused the FARC, Colombia's largest guerrilla group, of seeking to turn the hostage's release into a "media show."
"They want the world to thank them for the humanitarian gesture," he said.
"I want to tell them clearly and forcefully: I am not going to allow Mrs Piedad Cordoba nor any other official of any kind to go get this man they've kidnapped," he said.
Cordoba is a former leftist legislator who has often acted as an intermediary in previous hostage releases by the FARC.
The guerrilla group renounced the practice of kidnappings for ransom before entering into peace talks with the government last year.
The last police and soldiers it was holding hostage were released from captivity in 2012.
But on Friday the FARC said it had captured an American, whom it identified as Kevin Scott Sutay, on June 20 and wanted to release him.
US Ambassador Michael McKinley said Saturday the American was a former marine who was in the country on a tourist visit and had nothing to do with the US military mission in Colombia.
Santos said the FARC was "flagrantly violating the commitment they made at the start of the (peace) talks."
The talks, which have been under way since November, had raised optimism about bringing an end to a near 50-year-old insurgency, Latin America's oldest.
But a flare-up in fighting in recent days has left 27 soldiers and rebels dead in clashes in various parts of the country.
Tensions also were exacerbated this week after a FARC unit offered to provide arms and fighters to farmworkers protesting coca eradication efforts in a border region near Venezuela.
"It dismays me to hear statements by the FARC suggesting they are ready to provide arms and to accompany protests with armed men," said Carlos Villegas, a Colombian representative to the peace talks.
"We have to send a message to Colombian society that we are all ready for reconciliation so that the armed conflict can come to an end," he said.
FARC negotiators in Havana have so far made no comment about the offer.
The unrest in the northeastern Catatumbo region has resulted in four deaths over the past month and a half.
The government has complained for weeks that the FARC has infiltrated the protesters, who are also demanding an area be set aside where farmworkers would have a form of self-government.
The land issues are currently on the agenda in the talks in Havana, which are scheduled to resume on Sunday after a break.