Colombia's largest left-wing rebel group has said for the first time that it is holding three US citizens who vanished when their plane crash-landed on 13 February in a remote part of the country.
The crashed plane was set alight
The Revolutionary Armed Forces of Colombia (FARC) accused the three of being CIA agents and said they would be held until the military stopped operations in southern Colombia.
A fourth American and a Colombian army sergeant who were also aboard the US Government Cessna plane were found shot dead at the jungle site in Caqueta province.
We can only guarantee the life of the three gringo officials if the Colombian army immediately suspends military operations and
overflights in the area
Washington is actively supporting the Colombian Government's operations against drug-running and rebel groups.
The FARC's statement, broadcast by Colombian radio, described the captives as "American officers".
"We can only guarantee the life and physical integrity of the three gringo officials in our power if the Colombian army immediately suspends military operations and overflights in the area," it said.
'Give us proof'
A White House spokesman denied that the men were attached to the CIA , saying they were department of defence contractors.
And he rejected the rebels' demand to stop military operations in the area.
"We will continue to work closely with Colombia and its government and its military and its police on how to combat the FARC," spokesman Ari Fleischer said.
Colombian Defence Minister Martha Lucia Ramirez said on Friday that her government was working with Washington to find the missing Americans and "hopefully be able to conduct a rescue operation".
The Washington Post has reported that President George W Bush has sent an additional 150 troops to Colombia to assist the search.
Mr Bush condemned the killing of the other two men.
"One man had a bullet hole in the back of his head - clearly an execution," he said this week. "So we are dealing with cold-blooded killers that need to be treated as cold-blooded killers."
The FARC still controls large parts of rural Colombia
The Colombian Government has offered a $345,000 reward for information leading to the safe return of the missing men who have not been identified.
Correspondents say that if the FARC's claim is confirmed it will mark the first time that US Government employees have been captured during Colombia's four-decade-long civil war.