CUCUTA, Colombia -- Steep mountains, thick jungles: U.S. government-contracted pilots are facing immense challenges and hazards as they fumigate drug crops at ground-hugging altitudes in their latest offensive in eastern Colombia.

But the pilots' missions are made even dicier because they are flying beyond areas cleared of rebels by army troops, two senior Colombian army commanders told The Associated Press. The practice, which they say exposes pilots to hostile fire, goes against tactics normally used in the anti-drug campaign -- and may have already cost one contractor his life.

Mario Alvarado, a native of Costa Rica who worked for an unidentified U.S. company, was killed Sunday when his plane crashed while fumigating coca crops in the rebel stronghold of Catatumbo. The U.S. State Department said the OV-10 apparently "was struck by hostile ground fire." Government troops were in Catatumbo, but nowhere near the pilot's route.

"Where the planes sprayed on Sunday, the troops weren't there. They were a five-minute helicopter ride away, or several days travel by foot," said Gen. Jorge Pineda, commander of the Fifth Brigade, based in the sprawling, red-brick town of Cucuta, the staging area for the offensive.

President Alvaro Uribe has vowed to wipe out cocaine production in Colombia, the world's main supplier. The fumigation campaign has cut coca cultivation in the country by one-third in only seven months, the United Nations reported last week.

Operations between the ground forces and the spray planes are normally coordinated, so the low-flying crop dusters don't wander over areas rife with guerrillas.

But the Colombian commanders said in interviews Wednesday that in the new offensive -- now called Operation Catatumbo after previously being dubbed Operation Holocaust -- standard practices seem to have been ditched as ground troops move more slowly over the steep mountainsides, which are often wreathed in clouds.

Gen. Arturo Suarez, commander of the elite U.S.-trained Counter-Narcotics Brigade, said the Catatumbo region -- located in the far northern reaches of the Andes near the Venezuelan border -- was much tougher to operate in than other areas.

"It's very slow, very difficult, very thick with jungle," he said.

Leftist rebels control much of Colombia's cocaine production, and are quick on the trigger when they spot one of the low-flying spray planes.

Suarez, whose brigade was trained by U.S. special forces and equipped with dozens of U.S. helicopters specifically to protect the fumigation planes, said the pilots were jeopardizing their lives by fumigating unsecured areas.

"If they go in areas where we're not present, then they're running a grave risk," he said.

Jim Foster, a spokesman for the U.S. Embassy, said he was unaware of any operational changes. "I haven't heard that the planes are in one place and the troops are in another," he said.

Adam Isacson, a Colombia expert with the Washington-based Center for International Policy, wondered if authorities are sacrificing the pilots' margin of safety for expediency.

"The officials need to cut costs and show results, but they're really putting these guys on the edge and not giving them the cover they need," Isacson said.

Which company, exactly, is flying the State Department planes in Catatumbo is a mystery. AP journalists on Wednesday saw several of the blue twin-engine OV-10s, looking like odd insects with their bulging canopies, sitting unattended in a far section of Cucuta's airport.

DynCorp, based in Reston, Va., is contracted by the State Department to fly the fumigation missions but has subcontracted out the Catatumbo offensive to another U.S. firm, said DynCorp spokesman Chuck Wilkins. He refused to name the company.

Wilkins referred all requests for information about operational changes to Washington.

The State Department did not respond to requests for comment on the changes.

John McLaughlin, director of aviation in the State Department's Bureau of International Narcotics, recently questioned the department's ability to run the fumigation program.

"Simply put: dodging trees and ground fire over jungle terrain at 200 mph is not diplomacy, and diplomats cannot be expected to fully comprehend the complexity of the task and the level of support required," McLaughlin wrote in an Aug. 4 internal memo, first reported Thursday by columnist Robert Novak.

State Department spokesman Adam Ereli on Friday defended the department's air operations. He hailed the dedication and courage of the pilots, whose missions have "resulted in decreasing amounts of drugs reaching our people."

The Catatumbo offensive is probably the most hostile environment the fumigators have faced. They have previously sprayed coca in the Amazonian lowlands and the hilly plains of Putumayo and Narino states.

The Catatumbo region is a jumble of mountains, where the Revolutionary Armed Forces of Colombia, known as the FARC, have been dug in for years.

Suarez said seven of his U.S.-trained soldiers have been killed in action since the offensive began a month ago.

"The guerrillas are consolidated here," he said. "It's a tough situation to confront."