BOGOTA, Nov 8 (Reuters) - U.S. cocaine prices have risen sharply and drug purity has slipped in the last nine months in a signal of the sustained impact of the multibillion-dollar war on drugs, the White House drug czar said on Thursday.

Washington has spent more than $4 billion in Colombia as part of a project to fight cocaine-traffickers and guerrillas, but Democrats debating funding are questioning whether the program has succeeded in slashing drug shipments to U.S. streets.

John Walters, U.S. national drug control policy director, said the U.S.-backed campaign and more cooperation between Colombia and Mexico had disrupted the flow of cocaine sufficiently to push up prices by 44 percent -- one sign of scarce supply.

"We have never had disruptions of this magnitude," Walters told reporters during a visit to Bogota.

He said law enforcement figures showed shortages of cocaine in 37 U.S. cities. Purity of cocaine has decreased 15 percent over the last nine months, according to U.S. figures.

Critics of Plan Colombia -- the U.S.-funded anti-narcotics drive in Colombia -- have often dismissed U.S. government reports of price rises and drops in cocaine purity, saying they represent only short-term trends and not a sustained fall in drug supplies to the street.

U.S. Drug Enforcement Administration chief Karen Tandy said in May that the price of cocaine had in the past risen by as much as 50 percent for as long as seven months but had always fallen back down again.

Most of Colombia's cocaine ends up in the United States.

Aided by U.S. funds, President Alvaro Uribe has reduced violence from a four-decade-old guerrilla war by sending troops to retake areas once under rebel control and negotiating the surrender of violent paramilitaries who once fought the rebels.

Democrats who control the U.S. Congress have proposed reviewing Colombia's aid package to reduce the emphasis on military funds and aerial spraying of coca crops and increase spending on economic development and alternative crop programs.

The United Nations reported earlier this year that while production of coca leaf used to make cocaine had decreased in Colombia, output of the drug still hovered around 600 tonnes. (Reporting by Patrick Markey)

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