Anti-junta group members were attempting to de-mine some areas, but the work was done "by hand with only rudimentary equipment and without any professional training", it added.
"We know from bitter experience that civilian deaths and injuries will mount over time, and the widespread contamination is already blocking people from returning to their homes and farmland," said Rawya Rageh, the group's senior crisis adviser.
Myanmar is not a signatory to the United Nations convention that prohibits the use, stockpiling or development of anti-personnel mines.
Its military has been repeatedly accused of atrocities and war crimes during decades of internal conflict.
Military violence against the Rohingya minority in 2017 sent an estimated 750,000 people fleeing into neighbouring Bangladesh, bringing with them accounts of rape, murder and arson.
In March, the United States declared that the violence against the Rohingya amounted to genocide, saying there was clear evidence of an attempt to "destroy" them.
The Gambia dragged Myanmar before the International Court of Justice in 2019, accusing the predominantly Buddhist country of genocide against the Muslim minority.
The Hague-based court is due to give its judgment on Myanmar's preliminary objections to the case later this week.
Following the coup that ousted Aung San Suu Kyi's government, the military has waged a bloody crackdown on dissent that a local monitoring group says has killed more than 2,000 people and seen almost 15,000 arrested.
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