Antonio Maria Costa, executive director of the Office on Drugs and Crime, called the situation in Myanmar, also known as Burma, "extremely alarming." Opium is the main ingredient for heroin.
The announcement is likely to increase international pressure on Myanmar's military junta, which last month sparked outrage with its violent crackdown on anti-government protests.
The UN official said the majority of opium fields in the southeast Asian country are concentrated along the borders with China and Thailand in eastern Myanmar, where the central government has little control.
Myanmar remains the second-largest opium producer after Afghanistan, which accounts for 95% of global production. Costa said the price of opium in Myanmar is about twice that in Afghanistan.
"Opium is still very much the drug of choice there," he said, adding that there is a high level of corruption that allows the opium to reach neighboring countries.
