The drug czar, John Walters, wrote that retail cocaine prices fell by 11 percent from February 2005 to October 2006, to about US$135 (euro99) per gram of pure cocaine. That's way below the US$600 a gram pure cocaine fetched in 1981, when the U.S. government began collecting data, and near the level it has been at since the early 1990s.

During the same period, analysis of data collected by the U.S. Drug Enforcement Administration showed that after a drop in 2005, levels of purity "have trended somewhat toward former levels," Walters said.

Walters made the disclosure in a January letter to Sen. Charles Grassley, the Republican co-chair of the Senate Caucus on International Narcotics Control. The Washington Office on Latin America, a think tank, obtained the letter and made it available to The Associated Press.

Rafael Lemaitre, a spokesman for the White House Office of National Drug Control Policy, told the AP that Walters would not comment on the letter, but Lemaitre described it as "an accurate reflection of our agency's thoughts on the issue."

Next Wednesday, President Alvaro Uribe is set to meet with U.S. President George W. Bush at the White House to discuss U.S. support for Plan Colombia, the anti-narcotics and counterinsurgent program that has cost American taxpayers more than US$4 billion since 2000.

U.S. officials have insisted that Plan Colombia is reducing the quality and availability of cocaine on U.S. streets. Colombia supplies 90 percent of the cocaine consumed in the United States.

But Grassley, in an e-mailed statement, said the letter is "all the proof that anybody needs" that the White House drug office "has gotten quite good at spinning the numbers, but cooking the books doesn't help our efforts to curb cocaine and heroin production and consumption."

The numbers cited by Walters contradict upbeat appraisals made by U.S. officials as recently in March _ two months after Walters' letter.

Price and purity estimates are a key barometer of cocaine availability. Dropping prices are an indication of robust supply or weakening demand, as is rising purity.

Several household and school-based surveys show that America's cocaine consumption has barely budged since 2000, even as drug use in Europe, which also impacts supply, has soared.

Rep. Jim McGovern, a Massachusetts Democrat, said despite the existence of the new estimates, senior U.S. Embassy officials provided him with older, more upbeat data during a March visit to Bogota.

"We've given this program a chance to work and clearly this is not producing the results we were promised," he said. "Cocaine is priced as low and purity is as high as it was before Plan Colombia began six years and $5 billion ago."

And despite a record fumigation of almost 550 square miles (1,425 square kilometers) in 2005, there was 26 percent more land dedicated to production of the plant used to make cocaine. The 2006 estimates are to be released in May.

In November 2005, Walters announced that cocaine prices had risen by 19 percent and purity had dropped by about the same. He said the development signaled that the United States had turned the corner in the drug war. Drug policy experts rejected his assertions at the time, and Grassley called for his dismissal.

Walters' latest letter to Grassley came in response to a request from the senator.