Amnesty International says it has evidence of a "scorched-earth campaign" targeting Rohingya villages in Myanmar.
The human rights charity says more than 80 inhabited sites in Rakhine state - including entire villages - have been set ablaze since late August.
Satellite images, eyewitnesses and images from the ground have indicated an "orchestrated campaign of systematic burnings", Amnesty says.
The charity notes that the extent of damage cannot be independently verified on the ground due to restrictions in place by officials, but suggests the fires "are likely to have burned down whole villages, forcing tens of thousands to flee in terror".
It also says it has received reports of security forces and vigilante mobs 'shooting people at random' as they attempt to flee.
Police and army in Myanmar - also known as Burma - have launched a violent crackdown in Rakhine state following attacks by Rohingya insurgents on police posts last month.
It is claimed that thousands of people in Rakhine state have been killed or tortured by security forces in recent weeks.
According to the UN, more than 370,000 Rohingya Muslims have been forced to flee Myanmar to neighbouring Bangladesh in recent weeks.
Amnesty International's Colm O'Gorman said: "The evidence is irrefutable - the Myanmar security forces are setting northern Rakhine State ablaze in a targeted campaign to push the Rohingya people out of Myanmar.
"Make no mistake: this is ethnic cleansing. There is a clear and systematic pattern of abuse here."
It comes as the UN has warned of an "acute shortage of humanitarian supplies" as hundreds of thousands of people arrive in Bangladesh from Myanmar.