Leftist rebels accused by the United States of seizing three Americans have made a "very grave error" and can expect a U.S. response, an American congressman said Thursday.

The Americans apparently were captured by the Revolutionary Armed Forces of Colombia, or FARC, after their U.S. government plane crashed in rebel territory Feb. 13, U.S. officials have said. They were on an intelligence-gathering mission.

The rebels executed a fourth American and Colombian soldier who were also aboard, Colombia's army chief has said.

"I don't think there's any question that this precipitous action by the FARC is going to meet with very strong retaliation," said Rep. Tom Davis, D-Va., a member of a visiting Congressional delegation.

"Precisely what happens is being discussed as we speak, but they've made a very grave error."

The State Department declined to say what steps could be taken.

"Any questions on the response to the hostage scenario cannot be addressed at this time until the whereabouts of missing crew members is ascertained," said Department spokesman Lou Fintor.

U.S. officials said they have appealed to the captors for proof of life of the Americans.

"We want to see them released safely," said Richard Boucher, another State Department spokesman. "We want to see their safety and well-being confirmed."

The Colombian army on Thursday offered a $345,000 reward for information leading to the safe return of the three.

Davis said the American who was killed -- whom U.S. officials have not identified -- had "a stellar career serving our military and working here in Colombia."

Rep. Mark Souder, R-Ind., said the killing of the American -- the first to die in Colombia's war while on official business -- would not intimidate Washington.

"When there's an execution of Americans who are on duty, it's not likely to lead to retreat," Souder said.

Davis, Souder and Rep. Jim Moran, D-Va. -- the third member of the delegation -- praised President Alvaro Uribe's get-tough policy with the rebels and pledged to continue backing Colombia's armed forces.

But Rep. James McGovern, D-Mass., who is also visiting Colombia but was not part of the delegation, called for a reduction in the military aid in favor of more investment in social programs. Colombia has received $2 billion in the last three years.

The aid was originally restricted to counter-drug efforts but Congress recently allowed Colombia to use the equipment and U.S.-trained troops to battle the insurgents.

Washington has deepened its involvement in Colombia's conflict in recent months, sending 70 U.S. Green Berets to train Colombian troops to protect an oil pipeline carrying crude belonging to Los Angeles-based Occidental Petroleum.

"More war will not bring peace. More war is not going to bring more justice, or economic development," McGovern said in an interview.

The FARC and a smaller rebel group have been waging war against a succession of elected Colombian governments for 39 years. About 3,500 people, mainly civilians, die in the fighting each year.