Mexico City ? Mexican President Vicente Fox sent a drug decriminalization bill back to Congress on Wednesday just hours after U.S. officials warned that the measure could encourage “drug tourism.”

Fox refused to sign the bill, 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.”

Fox’s spokesman Tuesday had called the bill “an advance” and pledged the president would sign it. The measure was passed Friday by Congress and it immediately drew a storm of criticism because it eliminates criminal penalties possession of small amounts of heroin, methamphetamine 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.