U.S. and Colombian authorities yesterday stepped up an intensive aerial search for an American military reconnaissance aircraft that disappeared Friday over a rebel-infested swath of dense jungle in southern Colombia, focusing their efforts on an area where peasants reported hearing the sound of a low-flying plane followed by an explosion.

The search of the mountainous section of Putumayo province near the border with Ecuador has produced no sign of the missing De Havilland RC-7 or its crew of five U.S. Army soldiers and two Colombian air force officers, who were on an anti-narcotics mission over territory controlled by Marxist rebels of the Revolutionary Armed Forces of Colombia, officials said. But Colombian police said peasants told them they had heard an explosion around 4 a.m. local time Friday shortly after a plane flew low over their remote village of Betano. U.S. officials said the plane disappeared between 4 and 4:30 a.m.

"We've just flown over the region where the plane supposedly crashed, but we didn't see anything," Colombian police Lt. Teodoro Avildano told the Reuters news agency by telephone from the town of San Miguel in Putumayo province. He said the entire area was a "red zone" ruled by the guerrillas and no ground searches have yet been conducted.

There was no immediate indication that the missing plane had come under fire from the rebels, who have often shot at Colombian planes on low-flying missions to eradicate illegal drug crops.

U.S. military officials said the five missing Americans include two captains, a warrant officer and two enlisted men from Fort Bliss, Texas. The officials refused to release their names or units, citing "the sensitive nature" of their mission.

Their four-engine, propeller-driven plane typically carries sophisticated infrared sensors and electronic systems capable of eavesdropping on communications on the ground. It reportedly was looking for cocaine-producing laboratories as part of an intensified U.S. effort to stem the flow of drugs from Colombia, which supplies about 80 percent of the world's cocaine and more than two-thirds of the heroin consumed in the United States.

The U.S. Southern Command in Miami, responsible for military operations in Latin America and the Caribbean, said more planes and personnel were being added to the search and that the operations would continue around the clock.

Spokesman Lt. Col. John Snyder said a growing number of Army, Air Force, Navy and Customs Service aircraft, including UH-1 Huey helicopters and P-3 patrol planes, were taking part in the search. The Joint Interagency Task Force in Key West, which runs military counter-drug operations in the region, has also joined the effort. Officials declined to specify how many aircraft were involved.

"We have a clear idea of the aircraft's flight plan and a clear idea of the tracking of the aircraft through radar up to the point when we lost communication," Snyder said.

He declined to say whether any signals have been received from tracking gear aboard the plane or whether any guerrillas have been detected in the area where it supposedly went down.

At the time the aircraft disappeared, there was a cloud ceiling of about 4,000 feet. The RC-7 usually flies such missions at altitudes between 5,000 and 25,000 feet. The plane had been scheduled to fly over the province for up to eight hours before returning to a Colombian military base in a mountainous area near Apiay, 45 miles southeast of the capital, Bogota.

Kovaleski reported from Miami.