The International Committee of the Red Cross is negotiating the release of a former U.S. Marine seized by Colombia’s FARC rebels over a month ago.

Kevin Scott Sutay, a 26-year-old from New York, was captured June 20 in the town of Retorno, near the Venezuelan border.

"At the moment we are making contact but there is nothing more concrete to say," Colombia-based Red Cross spokesman Edgar Alfonso told Reuters. He added that the organization would only give further details about Sutay's release once it had taken place.

The news of the capture came out on a statement posted Friday on FARC’s website. It said Sutay described himself as a 2010-11 veteran of the Afghan conflict who left the armed forces in March.

“The capture of the soldier Kevin highlights the active participation on the field of U.S. mercenaries in military and counter-insurgency operations in which they appear under the euphemism of contractors, privatized form of imperial intervention forces, typical of the era of capitalist globalization.

U.S. Ambassador Michael McKinley denied allegations that Sutay was a mercenary and said he is not an active member of the military.

"He is a citizen who has nothing to do with Colombia's internal conflict," he said Saturday.

The chief of Colombia's anti-kidnapping police, Gen Humberto Guatibonza said Sutay wouldn't heed the advice of workers of the hotel he was staying at, San Jose del Guaviare, about 173 miles from the capital, Bogotá. Reuters reports he openly discussed plans to travel to the hamlet of El Retorno by navigating through the jungle with a GPS device and using survival skills learned in the military. Alarmed, the hotel workers notified the police and had him sign a statement saying he took responsibility for his action.

Meanwhile, President Juan Manuel Santos promised decisive retaliation Sunday after Colombia's main rebel band killed 19 soldiers in a single day in the biggest blow to the military since peace talks began in November.

"Just as we have extended our hand and are in negotiations, so do we have a big stick. We have decisive military force and will apply it," he said.

Santos traveled to Arauca state on the Venezuelan border, where 15 members of an army battalion that guards oil facilities were killed in an ambush Saturday.

The other four soldiers killed in combat Saturday died in the town of Doncello in the southern state of Caqueta, a traditional stronghold of the Revolutionary Armed Forces of Colombia.

The attacks fell on Colombia's independence day.

The FARC have been badly battered militarily in recent years and analysts say chances of ending its nearly half-century-old insurgency have never been better.

Both sides say they have reached a preliminary agreement on land reform. But they also say nothing is settled until everything on the six-point agenda is agreed upon.

The Associated Press contributed to this report.

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