Colombia's coca crop — the basis for cocaine — grew by 27 percent last year, the United Nations reported Wednesday, calling the increase "a surprise and a shock" given major U.S.-funded eradication efforts.

Eradication of the crop in Colombia, the world's No. 1 cocaine-producing nation, has been the cornerstone of a multibillion-dollar U.S. aid package.

Coca cultivation was also up 4 percent in Peru and 5 percent in Bolivia, the U.N. Office on Drugs and Crime reported in its annual survey.

It estimated cocaine production in the Andean region was stable, however, at about 994 metric tons compared to 984 metric tons in 2006.

Colombian cocaine production failed to keep pace with coca planting because police pressure forced an interruption in the growing cycle, Gen. Oscar Naranjo, chief of Colombia's police, told reporters.

"These young crops, the new ones, are less productive," Naranjo told a press conference in Bogota.

He said that eradication programs also are pushing coca farmers to more remote areas where it is harder to obtain the chemicals needed to make cocaine,

"The increase in coca cultivation in Colombia is a surprise and shock: a surprise because it comes at a time when the Colombian government is trying so hard to eradicate coca; a shock because of the magnitude of cultivation," the UNODC's executive director, Antonio Maria Costa, said in a statement.

He noted, however, that almost half of Colombia's coca comes from just 10 of the country's 195 municipalities. "Just like in Afghanistan, where most opium is grown in provinces with a heavy Taliban presence, in Colombia most coca is grown in areas controlled by insurgents."

In all, 99,000 hectares, or 382 square miles, of coca cultivation were found in Colombia last year, up from 78,000 hectares in 2006, the U.N. said. It estimated total cultivation last year in Colombia, Peru and Bolivia — the world's three principal sources of coca — at 181,600 hectares, or 701 square miles.

Last year, Colombia's drug police sprayed herbicide on 160,000 hectares of coca and manually eradicated another 50,000 hectares.

Washington has spent more than US$5 billion in Colombia over the past seven years to combat both a long-running insurgency and the world's largest cocaine industry, a business that helps fund a five-decade armed conflict.

Some Democrats in the U.S. Congress are criticizing the heavy military focus of U.S. aid to Colombia. About 80 percent of the money goes to the military while only 20 percent is dedicated to social projects designed to wean farmers off coca.

One reason for the rise in coca cultivation is that farmers are quickly replanting and minimizing the effects of aerial spraying in other ways — such as developing hybrids and coating coca plants with cane syrup that protects them from sprayed herbicides, said Bruce Bagley, a professor of international studies at the University of Miami.

"Areas that have been sprayed have then been brought back into production," he said. "It's time for aerial spraying to give way to other programs."