The domestic war has added to an estimated 346,000 people displaced before the coup.
Post-coup violence has pushed the number of displaced people in Myanmar over one million for the first time, the UN has said, warning of "dire" conditions as the monsoon approaches and fighting rages.
Diplomatic efforts to end the crisis have failed again after Russia and China blocked on Thursday the UN Security Council resolution against Myanmar junta.
Almost 700,000 people have been forced to flee their homes since the toppling of Aung San Suu Kyi's government last year, the UN Office for the Coordination of Humanitarian Affairs said on Thursday.
Civilian militias have formed to fight back against the coup across the country, and the junta has responded with an onslaught that rights groups say includes razing villages, mass extra-judicial killings and airstrikes on civilians.
The violence has added to an estimated 346,000 people already displaced before the coup.
That includes those affected by long-running conflicts with ethnic rebel groups along the Thai and Chinese borders, and Rohingya Muslims forced from their homes during a brutal 2017 crackdown.
More than 12,000 civilian properties are estimated to have been burned or destroyed since the putsch, UNOCHA said, with the approaching monsoon rains threatening more misery for those living in displacement camps.
More than 300,000 of those displaced since the coup were from the northwestern Sagaing region, where fighters clash regularly with junta forces, it added.