Myanmar police have arrested one American and two locals after photos on Facebook led them to a huge plantation of towering marijuana plants near Mandalay.

Pictures of the fields of weed started circulating on the platform last week -- a rare sight online in a country where police photos of seized heroin and methamphetamine are far more common.

Police raided the 20-acre site in Ngunzun township Monday to find nearly 350,000 marijuana plants -- some up to two metres tall -- 380 kilograms of seeds and 270 kilograms of marijuana, the Central Committee for Drug Abuse Control (CCDAC) announced Wednesday.

A released photo showed arrested US citizen John Fredric Todoroki, 63, standing alongside Myanmar nationals Shein Latt, 37, and Ma Shun Le Myat Noe, 23.

Another man, 49-year-old Alexander Skemp Todoroki, is still "at large", the CCDAC said.

Police confirmed he is also American.

The detainees have been charged under the Anti-Narcotics Drug and Psychotropic Substances Law, though it remains unclear what penalties they will face if found guilty.

"We didn’t know this (marijuana plantation) existed," one local police officer told AFP, asking not to be named.

"We only found out when we were tipped off about it." 

Seizures of heroin, pills and crystal meth by authorities are more common in Myanmar, where weak rule of law and conflict-riddled border areas allow for the industrial-scale production of harder drugs. 

Reaction on Facebook was swift, with some offering high praise for the arrests. 

Others questioned how the pot growers had been able to get away with it for so long.

"How could the plants have grown so big without you allowing it?" Facebook user Kg Zoe Law commented at the police.

But not everyone's nose was put out of joint by the agronomists' antics.

"Let me know where it'll be burned so I can get in position," San Yu Ko Ko pleaded.

© AFP