Wholesale prices of cocaine have risen in more than a dozen major U.S. cities as supplies of the powerful drug have shrunk, including in high-volume markets such as Los Angeles and New York, White House drug czar John Walters said.

But the same federal report that Walters said indicates the short supply also suggests that producers might be stockpiling cocaine in South America or Mexico, perhaps waiting until the time is right to export to the United States.

The scarcity was first noted in a May report by the National Drug Intelligence Center, said Walters, who heads the Office of National Drug Control Policy.

"We've had a number of programs and a number of efforts that have done things at a level that we haven't seen before," he told The Associated Press on Thursday.

The shortage is likely the result of combined enforcement and eradication efforts in the U.S., Mexico and Colombia, the world's largest producer of cocaine, said Walters, who met with intelligence officials in El Paso this week.

The shortage has been tracked largely by sharp price increases and has existed for about two to four months, according to a four-page unclassified summary of the NDIC's findings.

There is also the possibility that cocaine once bound for the U.S. has been redirected to Europe, the report shows.

Sales of other, cheaper drugs may also be climbing as cocaine becomes less affordable. In Ohio, Youngstown Police Chief Jimmy Hughes said officers noticing the recent cocaine shortage also are picking up on other drugs becoming more popular.

Youngstown is one of 26 cities the NDIC report says has had recent cocaine shortages and higher prices.

"We are seeing a lot of replacement drugs," Hughes said. "We're seeing more heroin ... and marijuana, as cocaine has been hard to get."

There is no evidence to suggest that increased U.S. consumption has led to the recent shortages, Walters said.

Colombian officials in April discovered about 27 tons of cocaine buried off the Pacific coast, the largest drug seizure in the country's history. Just a month earlier, the U.S. Coast Guard seized about 38,000 pounds of cocaine off the coast of Panama.

Brief cocaine price surges are not uncommon, said Peter Reuter, a public policy professor at the University of Maryland who studies illicit drugs and organized crime.

"We see short term (price) increases that go on for three, or six months even," Reuter said. "They don't tend to be too long, and then the downward trend continues. I think if it was more than six months, I would start to take a look at it."

The cities named in the report are Albany, N.Y.; Atlanta; Boston; Cleveland; Detroit; Grand Rapids, Mich.; Indianapolis; Los Angeles; Memphis, Tenn.; Milwaukee; Minneapolis; Nashville, Tenn.; New Haven, Conn.; New York; Philadelphia; San Francisco; and Youngstown