Opium cultivation and production have reached record levels in Afghanistan, triggering a large increase in cheaper heroin supply in the US, the United Nations said.
Afghan poppy fields covered some 224,000 hectares (553,500 acres) in 2014 -- a 7% rise from the 209,000 hectares the previous year, according to a new study by the UN Office on Drugs and Crime (UNODC).
As the world's largest opium producer, Afghanistan's steady increase has had a direct impact on global opium cultivation, which with 310,891 hectares is now at its highest since the late 1930s, UNODC noted in its annual World Drugs report.
Estimated global production of opiates meanwhile doubled in two years from 3,700 tons in 2012 to 7,554 tons in 2014.
Afghanistan accounted for 85% of the global output.
As a result, "we have been seeing signs in the US and the UK of a large increase of supply," UNODC research branch chief Angela Me said.
"In the US there's been an increase in heroin-related deaths and also signs of more and cheaper heroin available. The same goes for the UK where heroin-related deaths have increased recently."
Me noted that while drug-related deaths in the US had primarily been linked to the misuse of prescription opioids over the past few years, heroin was now "almost replacing" the trend.
The number of Americans dying from heroin abuse rose from 5,925 in 2012 to 8,257 in 2013, reaching the highest level in a decade, the UNODC report said.
Globally, some 32.4 million people -- or 0.7% of adults -- are users of opiates like heroin and opium.
Asia remains the world's largest market for opiates, accounting for an estimated two thirds of all users, while the number of registered heroin users in China is increasing, UNODC said.
The agency also highlighted the "increasing importance" of Africa as a transit hub for Afghan heroin bound for Europe and other regions, reflected in the growing figure of seizures being reported in recent years by some African countries.
But although heroin seizures increased by eight percent in 2014, "we have not seen big signs of increased supplies in terms of seizure", said Angela Me.
This suggests that traffickers may be seeking out new smuggling routes.