Some Colombian drug growers are using genetically modified coca "trees" to boost cocaine production dramatically, government officials say.

Anti-drug operatives say they found new strains with yields eight times higher than normal coca plants.

Higher yields could help explain why cocaine prices have stayed low despite US and Colombian air attacks on farms.

Colombian scientists and US officials expressed doubts, claiming extra growth could be achieved using fertiliser.

The coca "trees" can stand over 2m tall (6ft 6in) and produce four times as much of the alkaloid active in cocaine, according to a dossier seen by Britain's Financial Times newspaper.

Although official Colombian figures claim that the area under coca cultivation has halved since 2000, evidence suggests that coca planters have managed to maintain a net level of cultivation.

German Manga, an assistant to the Colombian vice-president, told the BBC that planters were using new and sophisticated technology to maintain their levels of production.