US "drugs tsar" John Walters has said the production of coca, the raw material for cocaine, has declined in Colombia by 30% in the past two years.
Walters gave a more upbeat message than last week
Mr Walters was speaking in Washington after visiting Colombia and Mexico.
He said American-backed efforts by those countries had sharply reduced the estimated flow of the drug to the US.
The statement appears to contradict comments he made last week. While in Mexico, he said there was no fall in the amount of cocaine reaching the US.
"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 at a news conference in Mexico City last Thursday.
In his latest remarks, Mr Walters, the head of the US Office of National Drug Control Policy, said 440 tons of cocaine had been seized last year in Latin America, the US and Europe.
He said this represented 40% of the estimated flow of the drug to the US and Europe.
"In the next 12 months, we will see changes in
availability of the drug - probably first lower purity,
followed by higher prices," he said.
Mr Walters said there were signs that cultivation and distribution of cocaine was beginning to be disrupted.
"We are at the beginning point," he said Walters. "No-one's claiming we have finished the job."
Washington is funding an aid package known as Plan Colombia, under which Colombian forces receive training, equipment and intelligence to root out drug traffickers and eliminate coca crops.
The plan has turned the country into the world's third-largest recipient of US military aid.
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.