US drugs tsar John Walters has admitted that Washington's anti-narcotics policy in Latin America has so far failed.
Walters praised Mexico's efforts to combat the drugs trade
Mr Walters said in Mexico that billions of dollars of investment over many years have failed to dent the flow of Latin American cocaine onto US streets.
"We have not yet seen in all these efforts what we're hoping for on the supply side, which is a reduction in availability," he said in Mexico City.
But he predicted positive results would be seen within a year.
Mr Walters was speaking just after he had visited Colombia, where US-backed efforts to wipe out drug-smuggling gangs and eradicate coca crops have turned the country into the world's third-largest recipient of US military aid.
However, in an interview with the Associated Press news agency, he defended the Plan Colombia aid package and insisted that it should continue.
"We have a history in the United States of not following through on programmes like this," he said.
While praising Mexico's efforts to combat the drugs trade, Mr Walters also said that there had been no notable disruption in the supply of drugs to the US.
Anti-narcotics raids had been successful in removing a number of important cartel leaders but had not led to any shortage.
However, Mr Walters said he hoped that there would be a decline in the amount of cocaine available in the United States over the next 12 months.
Correspondents point out that whenever the US has been able to cut coca production anywhere in Latin America, the shortfall has been made up by increases elsewhere in the region.
Production has notably risen in Peru, the world's next biggest producer of cocaine after Colombia.