Military officials in Colombia say fierce fighting is taking place with left-wing rebels who are holding three US Government employees hostage.
The Americans' plane had crash-landed in the jungle
A Colombian army officer said that, according to intelligence reports, soldiers were closing in on the guerrilla group holding the men.
US President George Bush is sending 150 extra troops to Colombia to help the search operation.
Colombia's largest left-wing rebel group, the Revolutionary Armed Forces of Colombia (FARC), said on Saturday it had the Americans, accusing them of being CIA agents.
If Americans get more deeply involved, it's going to get worse
Washington has denied the men were CIA agents, and said they were contractors for the defence department.
Colombian politicians on Sunday expressed concern at Mr Bush's decision to send in US troops.
"This is not Afghanistan, this is not Iraq, this is not Vietnam," said Congressman Gustavo Petro, a former guerrilla.
"If Americans get more deeply involved, it's going to get worse."
There are several hundred US military personnel in Colombia, including some special forces, but they are not allowed to take part in combat.
The US has spent $2bn in recent years to help Colombia tackle its illegal drugs trade, and recently lifted restrictions stopping the use of that aid against the guerrillas.
The three Americans held by FARC vanished when their plane crash-landed on 13 February in the remote southern province of Caqueta, long a rebel stronghold.
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.
FARC has said the three hostages would be held until the military stopped operations in southern Colombia.
"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 on Saturday, in a statement broadcast by Colombian radio.
This is the first time that US Government employees have been captured during Colombia's four decades of civil war.
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.