A study released Wednesday by the U.S. Office of National Drug Control Policy found Bolivia's coca production "statistically unchanged" from 2005, the year before the leftist president, who rose to prominence as a coca growers' representative, was inaugurated.

But U.S. officials say they still expect Bolivian cultivation of the plants used to make cocaine to rise under Morales' "zero cocaine, not zero coca" policy, continuing an upward trend that began early in the decade.

The U.S. report estimated Bolivia's coca plantings at between 21,000 and 32,500 hectares (52,000 and 80,000 acres), a range similar to 2005 estimates, the study said.

However, the report said those estimates were "not readily supportable," citing the difficulty in mapping Bolivia's largest coca-growing region, a forbidding maze of steep Andean foothills known as the Yungas.

"I think we're frankly quite concerned that there's been a buildup in cultivation," James O'Gara, the U.S. drug control agency's deputy director for supply reduction, told The Associated Press.

The Bolivian government does not keep its own statistics on total coca cultivation. Government officials said they had not received a copy of the report and declined to comment.

The United States in past years announced a single hard number for the notoriously hard-to-measure coca crop each year. In 2005, U.S. officials estimated Bolivia's coca production at 26,500 hectares (65,500 acres).

"They seem to be admitting here that coca cultivation estimates are not an exact science," said Kathryn Ledebur, director of the Andean Information Network, a nonprofit organization monitoring the U.S. drug war in Bolivia. "They've pretended for a very long time that it was."

Since taking office in January 2006, Morales has implemented a U.S.-funded coca-limitation plan negotiated with growers, but the U.S. report criticized his recent call to increase Bolivia's legal production limit from 12,000 hectares (29,652 acres) to 20,000 hectares (49,420 acres).

Morales considers the coca leaf a sacred Andean symbol and has promoted its traditional uses as a mild stimulant and treatment for altitude sickness. He fought past U.S. efforts to stamp out the crop.