The plane was carrying 116 people and it went missing between the southern city of Myeik and Yangon.
"Communication was lost suddenly at about 1:35 PM (07:05 GMT) when it reached about 20 miles west of Dawei town," the commander-in-chief’s office said in a statement, adding search and rescue was ongoing.
The plane was carrying 105 passengers and 11 crew.
“Now they have found pieces of the damaged plane in the sea 136 miles (218 km) away from Dawei city,” said Naing Lin Zaw, a tourism official in Myeik city, adding that the navy was still searching the sea.
Several navy ships and air force planes were sent to search for the aircraft, which was flying at an altitude of more than 5,486 metres.
The plane was a Y-8F-200 four-engine turboprop, a Chinese-made model still commonly used by Myanmar's military for transporting cargo.
Image used is for representational purposes only.
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