Myanmar police say they have seized a huge haul of liquid fentanyl, the first time one of the dangerous synthetic opioids that have ravaged North America has been found in Asia's Golden Triangle drug-producing region.

In a signal that Asia's drug syndicates have moved into the lucrative opioid market, Reuters can reveal more than 3,700 litres of methylfentanyl was discovered by anti-narcotics police near Loikan village in Shan State in northeast Myanmar.

The seizure of the fentanyl derivative was part of Asia's biggest-ever interception of illicit drugs, precursors and drug-making equipment, including 193 million methamphetamine tablets known as yaba. At 17.5 tonnes, the yaba almost equalled the amount seized in the previous two years in Myanmar.

The UN Office on Drugs and Crime (UNODC) said the scale of the bust was unprecedented and Myanmar's anti-drug authorities had "dismantled a significant network" during a two-month operation involving police and military. Also seized were almost 163,000 litres and 35.5 tonnes of drug precursors, as well as weapons. There were more than 130 arrests.

Even so, the methylfentanyl discovery was an ominous indicator for the region's illicit drug market, the UN agency said. Fentanyl itself is 25 to 50 times stronger than heroin.

As well as being easier and cheaper to produce than heroin, strong synthetic opioids like fentanyl can be readily concealed and transported as only small amounts can deliver thousands of doses.

"We have repeatedly warned the region fentanyl could become a problem but this is off the charts," said the UNODC's Southeast Asia and the Pacific representative Jeremy Douglas. "It is the shift in the market we have been anticipating, and fearing."