The Bush administration said Saturday it holds the Colombian rebels holding three Americans responsible for their well-being and demanded the hostages' immediate release.

State Department spokeswoman Amanda Batt said the government has learned that the Americans, among the five people aboard a crashed U.S. aircraft, are held by the leftist Revolutionary Armed Forces of Colombia, known by its Spanish acronym FARC.

"Those holding them captive are responsible for their safety, health and well-being. All available resources are being used around the clock to conduct search and rescue operations," Batt said. "We demand their immediate and safe release."

Pentagon officials said Saturday that President Bush has ordered an additional 150 U.S. soldiers to Colombia to help search for the three.

The FARC, Colombia's largest rebel group, said it was holding them and that it had shot down the plane, which crashed in southern Colombia on Feb. 13. A fourth American and a Colombian army sergeant were shot and killed at the crash site.

Bush used his authority to go beyond congressional limits on the number of U.S. troops in Colombia in sending the additional soldiers, the Defense Department officials said.

In 2001, Congress limited the number of U.S. troops in Colombia to 400 but allowed the president to exceed that number for emergency searches and rescues. The extra 150 troops brought the total U.S. forces in Colombia to 411, according to The Washington Post, which first reported Bush's order Saturday.

But White House spokesman Ari Fleischer said in Crawford, Texas, that the president's actions in sending additional U.S. forces were "within the congressional authorization." He declined to discuss details.

"We are working closely with Colombia," Fleischer said.

"FARC is an organization that has wreaked havoc on the Colombian economy and the Colombian people, and we have common cause in defeating FARC. And we will do that together," Fleischer said.

In a statement released Saturday, after Bush sent the additional troops, the FARC demanded that the Colombian military suspend operations in the area where the Americans disappeared. It accused the American hostages of being agents of the CIA.

U.S. defense officials have identified the men as contractors for U.S. Southern Command, the military command that oversees operations in Colombia and elsewhere in South America. Officials have said they do not work for the Central Intelligence Agency.

Batt said the United States' single goal now is to find and rescue the hostages. "Any questions about the U.S. response to that type of hostage scenario will not be addressed until the whereabouts and well-being of the missing crew members has been ascertained," she said. "We do not want to speculate on this. We have not authorized or requested any group to negotiate."

The kidnapping marks the first time that U.S. government employees have been captured in Colombia's 38-year civil war, in which about 3,500 Colombians are killed each year.

The United States has added the FARC, the other main leftist rebel group in Colombia and a right-wing paramilitary group fighting the rebels to its list of international terrorist groups. The United States says all three groups are financed by the drug trade.

The State Department has appealed to the captors for proof that the Americans are alive. The Colombian government on Thursday offered a $345,000 reward for information leading to the safe return of the three.

The United States has given Colombia about $2 billion, mostly in military aid, since 2001. The 2003 budget recently passed by Congress adds another $500 million.