MEXICO CITY, Mexico (AP) -- 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.

"Without underestimating the progress made on the issue, and with sensitivity toward the opinions expressed by various sectors of society, the administration has decided to suggest changes," according to a statement from his office.

Fox said he 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."

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 for possession of small amounts of heroin, methamphetamines and PCP, as well as marijuana and cocaine.

Weighing in, the U.S. government Wednesday 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."

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.