Colombia's coca crop increased by 8 percent last year despite a U.S.-backed eradication campaign that was the largest in history, U.N. drug officials said Tuesday.

Production of the plant used to make cocaine increased to 330 square miles in 2005 -- even as authorities sprayed coca fields totaling 25 times the size of Manhattan -- according to the U.N. Office on Drugs and Crime.

The U.N. findings follow an April report by the U.S. Office of National Drug Control Policy that reported a 26 percent increase in Colombia's coca production from 2004 to 2005, though that was partly due to a near doubling of the area surveyed.

Taken together, the reports are the strongest evidence yet that a cornerstone of the U.S.-led war on drugs -- the aerial fumigation of coca fields -- is failing to meet its goal of halving coca production in the Andes.

The anti-drug strategy known as Plan Colombia received about $4 billion in U.S. funding since 2000.

Colombia, the world's largest cocaine producer, is believed to be the source of 90 percent of the cocaine consumed in the United States.

The biggest increases were observed in the largely uninhabited jungles near Colombia's borders with Venezuela and Ecuador, the U.N. report said.

Key members of the U.S. Congress and growing numbers of Colombians have suggested it may be time to halt the potentially environmentally harmful spraying.

Maria Costa, executive director of the U.N. Office of Drugs and Crime, said the United States and Europe must curb their appetite for cocaine and increase support for alternative crop development programs in South America.

"Our aid efforts need to be multiplied at least tenfold in order to reach all impoverished farmers who need support," he said in a statement.