The drug-monitoring agency, International Narcotics Control Board (INCB), expressed its deep concern about the installation of vending machines in Los Angeles that sells medical marijuana.
"We know that the use of cannabis is illegal under federal law of the United States and we trust the authorities will stop such activities, which contravene the international drug control treaties," INCB president Philip Emafo said in the statement.
The Los Angeles Daily News reported in late January that patients with a special authorisation could obtain up to one ounce (28 grammes) of marijuana per week from special vending machines located in and around Los Angeles.
But the INCB, which monitors the implementation of UN drug conventions, noted: "So far, the results of research regarding the potential therapeutic usefulness (of cannabis) have been limited."
California is one of 11 US states to allow the use of marijuana for medical purposes, it added.