The U.S. State Department said three other people in the aircraft, all Americans, may have been taken hostage by the Revolutionary Armed Forces of Colombia, or FARC.
"We have reliable reports that crew members are being held by the terrorist group the FARC," State Department spokesman Charles Barclay said Friday in Washington. "If these reports are accurate, we demand the crew members be released unharmed immediately."
The bodies of an American and a Colombian were found in the wreckage of the plane. Gen. Jorge Mora, chief of the Colombian armed forces told reporters both were "executed, in an act of extreme cruelty." Both died from the gunshot wounds, said Alonso Velasquez, director of the attorney general's office in Florencia.
The identities of those aboard haven't been released.
The single-engine Cessna plane had gone down Thursday in rebel territory in southern Colombia. Three other occupants of the plane are missing.
The U.S. Embassy said the plane had crashed after experiencing engine trouble.
Uribe lamented what he called the deaths of "two people aboard the plane — a sergeant in our army and an American citizen — whose murder has been confirmed in the south of the country."
The Colombian president made his comments in a speech inaugurating a hydroelectric plant in western Colombia. He did not elaborate on his statement on the deaths.
It was unclear if the two men had been hit by groundfire while in the plane, or had been shot after the crash.
CBS News State Department Reporter Charles Wolfson reports the plane carried four U.S. citizens and one Colombian. All were involved in a counter-narcotics operation.
The Americans were contractors for the U.S. military's Southern Command, which oversees military operations in Latin America and the Caribbean, U.S. government officials said in Washington. The U.S. Embassy in Bogota said the plane crashed eight minutes before its scheduled arrival in Florencia, a provincial capital.
Colombian troops and U.S. officials continued their desperate search Friday for the survivors.
Authorities feared they had been captured by the Revolutionary Armed Forces of Colombia, or FARC, the nation's largest leftist rebel group.
If the survivors were captured, it would mark the first time in Colombia's decades-long civil war that Americans on U.S. government business had been taken by the insurgents.
Four Colombian soldiers involved in the rescue effort were reported injured by rebel land mines.
"The rebels have a large part of the area mined to stop troops from coming in," said Capt. Lida Zambrano, spokeswoman for the Colombian army's 12th Brigade.
Army troops patrolled the main road in the region, hoping to intercept the rebels if they tried to move the men out of the area by road. The army also closed the road between the towns of El Doncello and Puerto Rico — near where the plane was believed to have crashed — for several hours late Thursday, local resident said.
"All night long we heard helicopters," said Janet Jimenez, 21, a clerk at a corner cafe in the village of La Esmeralda. "There are a lot of army troops here."
The U.S. Embassy would not identify those aboard the plane, or say who they worked for or why they were in the region, an area largely controlled by the FARC. Plantations of coca — the main ingredient of cocaine — are prevalent in this region of humid plains and jungle-covered mountains.
The United States has backed a massive campaign to locate and destroy the drug crops with aerial fumigation.
Washington is now moving beyond simply fighting drug trafficking which provides profits for rebels and right-wing militias — to helping the Colombian government directly battle the insurgents.
U.S. special forces in eastern and central Colombia are training Colombian army troops in counterinsurgency tactics and Washington is planning to share intelligence on the rebels with Colombia. Dozens of companies have contracts with the U.S. government to maintain radar stations that track drug flights, fly crop-dusting planes that destroy drug crops and provide other services to Colombian security forces.
Some of the contractors work at the Larandia military base, near Florencia.
The FARC and the National Liberation Army have fought the government for nearly 40 years. About 3,500 people, mostly civilians, die each year in the fighting.