Hundreds of Colombian soldiers and U.S.-donated Black Hawk helicopters scoured rebel territory Saturday for three Americans allegedly kidnapped by a rebel group after their plane crashed in the region while on a spy mission, the army said.

Rebels shot and killed a fourth American and a Colombian army sergeant who were also on the plane, the country's top military commander, Gen. Jorge Mora said.

The U.S. aircraft was on an intelligence mission when it went down Thursday in a drug-producing area crawling with fighters from the Revolutionary Armed Forces of Colombia, or FARC. The pilot of the single-engine Cessna reported engine trouble shortly before the crash.

The United States has pumped almost $2 billion in mostly military aid into Colombia in the past three years, support that the FARC has called an act of war. The rebels have threatened to target American officials and interests.

Colombia's government has long favored a stronger U.S. role here and analysts speculated Saturday that officials may use the killing and kidnapping of the Americans -- along with two deadly bomb explosions blamed on the rebels -- to lobby for even more help from Washington.

The State Department said it had "reliable information" that the FARC had kidnapped the three surviving Americans. The 16,000-strong rebel group is waging a 38-year insurgency against the Colombian government.

On Saturday, 1,000 Colombian soldiers assisted by U.S.-made helicopter gunships -- normally reserved for drug-fighting missions -- searched for the Americans near the village of Doncello in Caqueta province 210 miles south of the capital, Bogota, army commander Gen. Carlos Alberto Ospina told The Associated Press.

U.S. officials were lending logistics support and intelligence information to the search effort, said Ospina.

The names of the Americans have not been released, and a U.S. Embassy official declined comment Saturday on the search.

Most of the U.S. military aid has been aimed at fighting the drug trade, which provides a huge source of income for both rebels and rival right-wing militias. Restrictions on that aid were recently lifted, allowing Colombia to use it to fight the rebels.

Washington stepped up its involvement in the country's civil conflict last month, sending U.S. Army special forces soldiers to train Colombian troops to fight insurgents in a violent eastern province.

President George Bush recently asked Congress for more than $500 million in additional aid for Colombia, and recent events could strengthen Colombia's argument for more help.

"They'll make the case that this is a critical moment," said Michael Shifter of the Inter-American Dialogue in Washington.

In recent days, Colombia has been rocked by some of its worst violence in recent memory. On Feb. 7, suspected FARC fighters detonated a bomb inside a prestigious social club in Bogota, killing 35 people and injuring more than 100 others.

Early Friday, 17 others were killed when a bomb exploded inside a house located under the flight path of planes landing at the airport in Neiva, about 250 miles south of Bogota.

FARC fighters had planted the explosives in the house and were planning to blow President Alvaro Uribe's plane out of the sky when it landed at the airport for a scheduled visit, authorities said.

Police were tipped off to the assassination plot and were searching the house Friday morning when the bomb went off. It was not clear what detonated it.

After arriving in Neiva around midnight Friday, Uribe visited with family members of the victims and later called the rebels "cowards."

"They're not capable of confronting the state head on," said Uribe. "They continue with this cowardly terrorism."