Colombian rebels declared Monday that three captured Americans were "prisoners of war" and will be freed only as part of a broad prisoner exchange with Colombian government.

The Revolutionary Armed Forces of Colombia, known as the FARC, demanded a demilitarized zone from the Colombian government in order to exchange the three Americans and dozens of Colombian soldiers and police -- who are also held by the rebels -- for insurgents who are in Colombian prisons. The rebels posted their statement on their Internet site.

The three Americans were captured on Feb. 13 after their U.S. government plane went down in FARC territory. A fourth American and a Colombian were shot and killed near the scene of the crash. Colombian authorities announced Monday they had captured a suspect in the killings.

"The three gringo prisoners of war in the custody of our organization will be liberated along with other Colombian prisoners of war once an exchange materializes in an large demilitarized zone," the FARC said in its statement.

President Bush authorized the deployment of up to 150 more troops to Colombia to help in the search for the three Americans, U.S. Southern Command spokesman Art Merkel said Monday.

The FARC said in its statement that the decision to send more troops to Colombia, where more than 200 military personnel are already training Colombian forces, was "a bald-faced invasion by the United States of our country."

Members of the Senate's foreign relations committee may call a special session to debate the deployment, said Jimmy Chamorro, vice president of the committee. The Senate is currently in recess.

The constitution requires that the Senate authorize any U.S. deployment that involves offensive operations, Chamorro said.

U.S. officials said Monday the additional troops would not be allowed to take on combat roles.

The U.S. government has not announced what specific mission the Americans were on when their plane went down over southern Colombia's Caqueta state. Officials said they were contractors with the Miami-based Southern Command.

Colombian authorities announced Monday that they had charged Fidel Casallas Bastos with homicide, kidnapping, possessing landmines and rebellion in the kidnappings and shooting deaths.

Bastos was captured near where the plane went down on Feb. 14, officials said. Prosecutors did not say what evidence they had to implicate Bastos.

The United States has been helping Colombia fight production of cocaine and heroin in this South American country. The drug trade is controlled by the rebels and a rival right-wing paramilitary group. Washington recently expanded its assistance -- including the training of Colombian troops by U.S. special forces and millions of dollars worth of helicopters and other gear -- to counterinsurgency aid.

The mountains and jungles of Caqueta state are a prime cocaine-producing region and a rebel stronghold. During three years of failed peace talks, the rebels controlled a vast demilitarized zone that encompassed part of Caqueta and Meta states. The government ended the peace talks and sent troops back into the region a year ago, but rebels still control much of the area.

The Colombian government had ruled out a prisoner exchange with the rebels, but recently said such a deal might be possible. However, President Alvaro Uribe has said it would never give the rebels another safe haven.