The International Committee of the Red Cross said Saturday it has made contact in Colombia with leftist guerrillas who said they wanted to release a captured US ex-soldier.
US ambassador Michael McKinley said the American, Kevin Scott Sutay, had nothing to do with the US military mission in Colombia and was in the country as a tourist.
"We demand that they free him as soon as possible," McKinley told reporters.
The Revolutionary Armed Forces of Colombia (FARC) said Friday it captured Sutay June 20 in the southern department of Guaviare.
It offered to release him as a gesture toward peace talks underway in Havana between the guerrilla group and the government.
"We are already in contact with the parties," a spokesman for the ICRC told AFP. "The ICRC is prepared to offer its good offices to facilitate this release."
The FARC said their captive served in the US Army from November 17, 2009, to March 22, 2013, including in Afghanistan in 2010-11.
McKinley, however, said he was a retired marine who was in the country on a tourist visit.
"He has nothing to do with the military mission, nothing to do with the armed conflict," he said.
Before entering into peace talks last year, the FARC pledged to end kidnappings for ransom of civilians and in April 2012 released the last 10 soldiers and police in captivity.
The rebel group, Colombia's largest with an estimated 8,000 fighters, has taken dozens of soldiers, police and politicians hostage over the course of its near 50-year-old insurgency.
Three US contractors -- Marc Gonsalves, Thomas Howes and Keith Stansell -- were held hostage from 2003 to 2008 after the aircraft they were flying crashed in rebel-held territory.
They were rescued in July 2008 in a Colombian army operation that also freed Ingrid Betancourt, a politician with dual Colombian-French citizenship.