SAN DIEGO ” Mexico’s crackdown on drug traffickers has helped cut supplies of cocaine in 37 U.S. cities and led to higher prices, the White House drug czar said Monday.

The disclosure came as the Bush administration prepares to present Congress with an aid package costing hundreds of millions of dollars to assist Mexico in fighting drugs, John Walters, director of the Office of National Drug Control Policy, told The Associated Press.

Walters declined to elaborate on the aid measures. The first part of the package is expected to be announced within days, he said.

“There has never been a better time” for such an aid package to Mexico, said Walters, who plans to tout the figures Tuesday at a meeting with federal, state and local authorities in San Diego.

A gram of pure cocaine cost $118.70 from April to June, up 24 percent from the $95.89 reported for the previous three months, Walters said, citing figures from the Drug Enforcement Administration.

That’s the highest price since the DEA began tracking data in the spring of 2005, when a gram fetched $93.63.

More recent figures were unavailable, but Walters believes prices kept rising through September.

Meanwhile, fewer workers tested positive for cocaine and fewer users turned up in emergency rooms, Walters said.

Quest Diagnostics Inc. performed more than 4.4 million workplace drug tests during the first six months of the year, reporting that 0.58 percent showed cocaine use, down from 0.69 percent in both 2005 and 2006.

Quest, the nation’s largest workplace drug testing company, said the most recent results were the lowest since it began collecting data a decade ago.

Dr. Steven Karch, a forensics expert in Berkeley, Calif., said he had not seen the figures but was skeptical about the decline in supply.

“I hope they’re right,” Karch said. “Every time we hear this, it turns out to be business as usual.”

Enforcement by U.S. and Colombian authorities partially explained the cut in supplies, but the government of Mexican President Felipe Calderon was the “most dramatic” change in recent months, Walters said.

About 90 percent of cocaine in the U.S. enters the country through Mexico, with much of it originating in Colombia.

“The big weight added in the last six months is Mexico,” Walters said. “Mexico has been doing things they’ve never done before.”

Calderon has deployed more than 20,000 soldiers and federal police agents in regions plagued by drug violence since he took office last year and extradited several high-level traffickers to the U.S.

U.S. law enforcement agencies first noted cocaine declines in the Northeast around March. Prices have nearly doubled in some Northeast and Midwestern cities, Walters said.

The 37 cities showing drops in cocaine supplies include Atlanta, Boston, Chicago, Denver, Detroit, Houston, Los Angeles and New York, according to law enforcement intelligence reports cited by Walters.