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Mexican President Vicente Fox refused to sign a drug decriminalization bill Wednesday, hours after U.S. officials warned the plan could encourage "drug tourism."

Fox sent the measure back to Congress for changes, but his office did not mention the U.S. criticism.

Fox will ask "Congress to make the needed corrections to make it absolutely clear in our country, the possession of drugs and their consumption are, and will continue to be, a criminal offense," according to a statement from the president's office.

On Tuesday, Fox's spokesman had called the bill "an advance" and pledged the president would sign it. But the measure, passed Friday by Congress, drew a storm of criticism because it eliminates criminal penalties possession of small amounts of heroin, methamphetamines and PCP, as well as marijuana and cocaine.

Earlier in the day, the U.S. government expressed a rare public objection to an internal Mexican political development, saying anyone caught with illegal drugs in Mexico should be prosecuted or given mandatory drug treatment.

"U.S. officials ... urged Mexican representatives to review the legislation urgently, to avoid the perception that drug use would be tolerated in Mexico, and to prevent drug tourism," U.S. Embassy spokeswoman Judith Bryan said.

There are concerns the measure could increase drug use by border visitors and U.S. students who flock to Mexico on vacation.

Bryan said the U.S. government wants Mexico "to ensure that all persons found in possession of any quantity of illegal drugs be prosecuted or be sent into mandatory drug treatment programs."

Jerry Saunders, mayor of San Diego — just a short drive from the border town of Tijuana, Mexico — applauded Fox's decision, saying he was "appalled" by the bill because it could increase drug availability north of the border.

"We have been a partner with Mexico in fighting against illegal drugs, and this will only help in the long-term in that relationship," he said.

The legislature has adjourned for the summer, and when it comes back, it will have an entirely new lower house and one-third new Senate members following the July 2 elections, which will also make the outgoing Fox a lame duck.

However, Sen. Jorge Zermeno, of Fox's conservative National Action Party — a supporter of the bill — said he thought Congress would be open to changing the legislation to delete a clause that extends to all "consumers" the exemption from prosecution that was originally meant to cover only recognized drug addicts.

"The word 'consumer' can be eliminated so that the only exemption clause would be for drug addicts," Zermeno told The Associated Press. "There's still time to get this through."

The bill contained many points that experts said were positive: it empowered state and local police — not just federal officers — to go after drug dealers, stiffened some penalties and closed loopholes that dealers had long used to escape prosecution.

But the broad decriminalization clause was what soured many — both in Mexico and abroad — to the proposal.

Mexico's top police official, Eduardo Medina Mora, acknowledged on Tuesday that the U.S. anti-drug agency has expressed concern about the law. Some senators and community leaders in Mexico also objected to the bill. But even if it had passed, he noted that Mexican cities have the power to impose fines and overnight jail detentions for those caught with drugs in public.

Current Mexican law allows judges latitude to drop charges if suspects can prove they are addicts and the quantity they were caught with is small enough to be considered "for personal use," or if they are first-time offenders.

The new bill would have made the decriminalization automatic, allowed "consumers" as well as addicts to have drugs, and delineated specific allowable quantities, which do not appear in the current law.

Under the law, consumers could have legally possessed up to 25 milligrams of heroin, a half a gram of cocaine and about one-fifth of an ounce of marijuana.