UN anti-narcotics chief said on Wednesday (12 November) that US states' move to legalize marijuana is not in line with international drugs conventions.
Residents across Oregon, Alaska and the US capital voted in favor of legalizing marijuana use this month.
"I don't see how (the new laws) can be compatible with existing conventions," Yury Fedotov, executive director of the United Nations Office on Drugs and Crime (UNODC), told reporters.
Fedotov will be discussing the increasing recognition of marijuana in American mainstream next week in Washington with the US State Department and other UN agencies.
Under US federal law, marijuana remains classified as an illegal narcotic.
The Obama administration, however, has allowed individual states to determine their own recreational-use statues.
Fedotov fears the US developments might be the start of a global trend of legalizing marijuana use.
In 2013, Uruguay became the first country to approve a bill to legalize and regulate the production and sale of marijuana.