Two men were arrested in the border town of Tachileik in eastern Shan State during the bust, which netted drugs with a street value of USD 2.3 million, police said.
"We seized 404 blocks of heroin weighing 140 kilogrammes," police colonel Myint Aung of the anti-narcotic department said. "It's the largest seizure of heroin so far this year."
Opium, heroin and methamphetamine pills are frequently smuggled from Myanmar into Thailand, the gateway to the Southeast Asian narcotics market.
The United Nations Office on Drugs and Crime (UNODC) says 10 per cent of the world's opium is produced in Myanmar, making it the second largest producer of the drug -- the base ingredient for heroin -- after Afghanistan.
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Myanmar has pledged to crack down on drug production and smuggling, despite the challenges posed by policing vast, remote areas where armed ethnic rebel groups are often accused of using profit from narcotics to fund their operations.
Authorities torched some 1.3 tons of seized opium, 225 kilogrammes of heroin and 1.2 tons of methamphetamine tablets last month to mark World Drugs Day.