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 shot and 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," the Revolutionary Armed Forces of Colombia, or FARC, said in a statement.

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, which has lasted nearly four decades.

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 Americans 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.

This marks the first time in more than 20 years that U.S. government employees have been killed or captured in the conflict, which pits the FARC and a smaller rebel army against the government and a handful of outlawed right-wing paramilitary groups. About 3,500 people, mostly civilians, die in the fighting every year.

The rebels kidnap hundreds of people a year in Colombia for money and political leverage. The FARC currently is holding about two dozen politicians and 45 soldiers and police officers, one of whom has been a hostage for more than five years.

The bodies of the two slain men were found about a mile from the plane, Carolina Sanchez, spokeswoman for the Colombian attorney general's office, said Saturday. The American had been shot in the head while the Colombia was shot twice in the chest. Autopsy reports showed that the bullets were fired from a distance of less than 2 yards, Sanchez said.

The rebels' demand specified that the military must halt operations and overflights in a 387-square mile area of southern Colombia's Caqueta state.

The 17,000-strong FARC considers U.S. involvement in Colombia an act of war and has warned that it would attack U.S. citizens and interests in the country.

Washington has given Colombia roughly $2 billion in mostly military aid in the past three years. The aid was initially limited to fighting drugs, but the restrictions recently were lifted to let Colombia use the equipment and U.S.-trained troops to confront the rebels directly.

U.S. troops are in the country training soldiers. The U.S. Congress in 2001 limited the number of U.S. troops in Colombia to 400, but allowed the president to exceed that number for emergency search and rescue operations.

Bush's order to deploy 150 more troops put the total in Colombia over the 400 limit, Defense Department officials said without giving an exact number.

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 their safe return.