KABUL, Afghanistan — For the third year in a row, opium cultivation has increased across Afghanistan, erasing earlier drops stemming from a decade-long international and Afghan government effort to combat the drug trade, according to a United Nations report released on Monday.

The report’s findings raised concerns among international law enforcement officials that if the trend continued, opium would be the country’s major economic activity after foreign military forces depart in 2014, leading to the specter of what one official referred to as “the world’s first true narco-state.”

Afghanistan is already the world’s largest producer of opium, and last year accounted for 75 percent of the world’s heroin supply. “The assumption is it will reach again to 90 percent this year,” said Jean-Luc Lemahieu, the United Nations’ top counternarcotics official here.

The report, the Afghanistan Opium Risk Assessment 2013, issued by the United Nations Office on Drugs and Crime and based on extensive surveys, found that opium cultivation had increased in 12 of the country’s 34 provinces. Herat, in western Afghanistan, is the only province in which cultivation is expected to decrease, the report said.