A U.S. Army reconnaissance plane on an anti-narcotics mission was reported missing and feared down in southern Colombia yesterday with five American soldiers and two Colombians on board.

American military authorities lost contact with the plane about 4:30 a.m. local time, three hours after it took off from a Colombian air base, apparently on a mission to take pictures of coca cultivation and scout for cocaine-producing laboratories.

A broad search by U.S. and Colombian aircraft, much of it over hilly and forested terrain heavily populated by Marxist-led guerrillas, failed to produce any sign of the propeller-driven De Havilland RC-7, U.S. military officials said. Low clouds were hampering the hunt, the officials said, but the effort was due to continue through the night.

It was the first reported disappearance of U.S. military personnel in Colombia, despite years of close cooperation between the Pentagon and Colombian authorities in combating the narcotics trade. One senior Pentagon official said that ground fire over territory inhabited by traffickers and Colombia's powerful insurgent groups has posed a constant threat to American patrol aircraft. Hostile action has brought down Colombian air crews and private American pilots who, working on contract for the U.S. government, fly crop dusters on fumigation runs to eradicate drug crops.

Colombia produces 80 percent of the world's cocaine and about two-thirds of the heroin consumed in the United States. It is slated to receive $300 million in U.S. counter-drug aid this year, making it the third-largest military aid recipient after Israel and Egypt.

The Pentagon has sought to limit U.S. assistance to anti-narcotics operations, but pressure has grown to assist Colombian authorities in their battle against the guerrillas. The leftist rebels have been vociferous critics of Washington's anti-drug role and have threatened to target U.S. personnel in Colombia.

Four months ago, the United States expanded its intelligence sharing operation in Colombia, fearing the government may be losing its counterinsurgency war. The new guidelines authorized for the first time the sharing of sensitive, real-time intelligence on the guerrillas with the Colombian military.

The two main guerrilla groups -- the Revolutionary Armed Forces of Colombia (FARC), with about 15,000 combatants, and the National Liberation Army (ELN), with about 5,000 combatants -- control about 50 percent of Colombia's territory. Colombian and U.S. intelligence reports say both insurgent groups derive much of their money -- up to $600 million a year -- from protecting drug traffickers, their routes and laboratories. They are also heavily involved in kidnapping.

Since taking office as president of Colombia last August, Andres Pastrana has tried to engage the guerrillas in sporadic peace talks. But the contacts, which were due to resume last Monday, were suspended indefinitely as the war flared anew with a rebel ambush on an army patrol in Bogota. Last week, FARC insurgents unleashed a 48-hour nationwide offensive in which about 300 people, mostly rebels, were reported killed.

A statement issued by the U.S. Embassy in Bogota, Colombia's capital, described the RC-7 flight yesterday as a "routine" mission. A senior Pentagon official said U.S. Army reconnaissance teams move in and out of Colombia frequently, deploying there for a week or two at a time.

The plane took off about 1:30 a.m. local time from a Colombian air base at Apiay, in the eastern plains region, for what military officials said was supposed to have been a seven- or eight-hour operation. The U.S. crew on board consisted of three officers and two noncommissioned service members. The Army declined to release any additional information about them.

After the plane was reported missing, U.S. authorities scrambled a Navy P-3 patrol plane out of Ecuador, and two more P-3s joined the search later, along with State Department anti-drug helicopters and an undisclosed number of Colombian military and police units, defense officials said. The Pentagon's national command center set up a crisis action team to monitor the search.

"There's still no information about what caused this plane to go down," a senior Pentagon official reported yesterday evening. But the official described the area in southern Colombia where the hunt was concentrated as "among the most FARC-infested parts of the country."

He expressed concern that even if the crew members had survived a fall from the sky, they faced grave danger on the ground.

"It's not where you want to be any time," the official said.

He added that search-and-rescue teams were themselves at some risk.

"The longer they're loitering around that area, the worse it is for everybody," the official said, noting that the presence of search aircraft could tip off insurgents to the location of the missing crew.

In February, three Americans working with a remote indigenous Colombian group were kidnapped by FARC guerrillas and were found slain two weeks later on the Venezuelan side of the Arauca River that borders Colombia. It was the first premeditated killing of U.S. citizens by the FARC in its 34-year-history. Colombian authorities have ordered the arrest of German Briceno, the brother of FARC's chief military strategist, Jorge Briceno, in connection with the killings.

The four-engine RC-7 is a versatile intelligence-gathering aircraft, specially designed to be deployed abroad for seven to 10 days without much ground support. The Pentagon has only five or six of the planes, all bought within the past eight years.

The plane flies at an altitude that can vary from 6,000 feet to 25,000 feet depending on whether the mission is primarily collecting images or listening in on communications signals. Its normal cruise speed is 220 knots, but it can loiter over an area at speeds as low as 110 knots.