World drugs body INCB on Tuesday warned Uruguay and the US states of Colorado and Washington that legalising the recreational use of marijuana violated international treaties.
In its annual report, the UN agency said it saw a "dangerous" trend in Uruguay after President Jose Mujica signed into law in December his government's controversial plan to oversee the production and sale of marijuana.
The International Narcotics Control Board, which monitors the implementation of international treaties on drugs, said Uruguay was in violation of the 1961 Single Convention on Narcotic Drugs.
The treaty prohibits the production and supply of specific drugs except under licence for specific purposes, such as research or medical treatment.
The INCB also deeply expressed regrets over "developments at the state level in Colorado and Washington, in the United States, regarding the legalisation of the recreational use of cannabis".
The Rocky Mountain state made drug history on New Year's Day with the inauguration of retail sales of marijuana.
The Pacific Northwest state of Washington is set to follow suit later this year -- even though, under federal law, marijuana remains as illegal as heroin, ecstasy and LSD.
The INCB called on the federal government in the US to "ensure that the treaties are fully implemented" throughout the country.
"When governments consider their future policies on that matter, the primary consideration should be the long-term health and welfare of the population, in the spirit of the preamble to the 1961 Convention," it said.