LA PAZ (AFP) - Four UN anti-narcotics workers and two Bolivian military pilots were killed when their plane crashed on a cocaine surveillance mission, a senior Bolivian official said Saturday. "It was a direct impact against a tree some 50 meters (164 feet) high and the plane completely burned," Bolivian Air Force General Tito Gandarillas told reporters. The Cessna plane left La Paz airport on Thursday to monitor coca growing in Los Yungas region, Bolivia 's main coca growing region northeast of the capital. The four staff worked for the UN Office on Drugs and Crime, which works with the government to counter narcotics smuggling. Air Force rescue helicopters found the crash site in an area some 180kms east of La Paz, between the provinces of Yungas and Inquisivi. "The place is... completely inaccessible, in a very tall forest," said Gandarillas. Gandarillas, who heads the Bolivian air force and was in charge of the search and rescue operation, said the bodies will be recovered later in the day. The crash site area, on the eastern slope of the Andes, is characterized by thick forests and deep ravines. Bolivia is the world's third largest producer of coca leaf - the source plant for cocaine - after Peru and Colombia, according to the UNODC.