Bogota, Colombia ? Leftist rebels on Saturday acknowledged for the first time that they shot down a U.S. plane and are holding hostage three Americans they accuse of being CIA agents. The White House sent 150 soldiers to join the search for the captives.

The Americans were on a U.S. government plane on an intelligence mission when it crashed on Feb. 13. A fourth American and a Colombian army sergeant were killed at the site.

“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,” Revolutionary Armed Forces of Colombia, or FARC, said in a release.

Gen. Jorge Enrique Mora, Colombia’s top military commander, rejected the rebel demand and said the search for the three Americans would proceed in the mountains and jungles of southern Colombia.

“We have hope that we’ll be able to get these three people back safe and sound,” Mora said, adding that Colombian counterdrug troops who have been trained by American Green Berets and U.S.-donated helicopters were being used in the search.

President Bush ordered an additional 150 U.S. soldiers to Colombia to help in the search, Pentagon officials said Saturday. U.S. officials already have been assisting with intelligence information.

The deployment will bring to more than 400 the number of U.S. troops in the South American nation, and will deepen Washington’s involvement in the civil war.

The rebels claimed they shot down the aircraft, contrary to assertions by the U.S. and Colombian governments that the single-engine Cessna went down because of engine trouble, and said the three men were CIA agents.

The Americans were on an intelligence mission when it crashed, Colombian Defense Minister Martha Lucia Ramirez has said. U.S. defense officials deny the Americans worked for the CIA and say the men were contractors for the Miami-based U.S. Southern Command, which oversees U.S. military operations in South America and the Caribbean.

The Bush administration demanded Saturday that the three Americans be freed and declared it holds the rebels responsible for their safety.