BOGOTA, Colombia — A U.S.-donated military helicopter crashed Thursday while flying in thick fog during a nighttime counternarcotics mission in southwest Colombia, killing all 20 soldiers aboard, the army said.

The Black Hawk helicopter, used in the U.S.-funded anti-drug effort called Plan Colombia, went down just after midnight near the village of Manguipayan, 340 miles southwest of the capital, Bogota.

Early investigations indicate the crash was caused by bad weather, the army said in a statement. An army official said the problem was "thick fog."

A U.S. official in Bogota, speaking on condition of anonymity, said the Black Hawk had been donated to Colombia as part of Plan Colombia. He said U.S. personnel were aiding in efforts to recover bodies of the victims and the wreckage of the chopper.

The Black Hawk was part of an assault mission that included seven other aircraft, the army statement said. The dead included six officers.

Army chief Gen. Reinaldo Castellanos told local radio the area is full of guerrillas from the Revolutionary Armed Forces of Colombia, or FARC. Although the rebels often fire on government helicopters, that likely wasn't the case in Thursday's crash, Castellanos said.

Castellanos said he was heading immediately from Bogota to the crash site.

The 20 killed was the highest death toll from a helicopter crash in Colombia since February 2003, when a Colombian army Black Hawk crashed in the mountains of northern Colombia, killing 23 crewmembers and elite troops. That crash also was caused by bad weather.

The FARC, which has been fighting the Colombian government for 40 years, uses drug trafficking to finance its insurgency.

Colombia's military began a major offensive last year aimed at wiping out the FARC and a smaller leftist group or forcing them to the negotiating table.

In the past five years, the United States has provided more than $3.5 billion in hardware, training and logistics planning toward Plan Colombia, which aims to wipe out drug production and trafficking in Colombia.

Since 1999 and including Thursday's crash, six U.S.-made Black Hawks have crashed in Colombia, killing at least 67 soldiers and injuring 37, according to records from The Associated Press.