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"After a careful and thorough analysis of available facts, it is clear that the situation in northern Rakhine state constitutes ethnic cleansing against the Rohingya," Secretary of State Rex Tillerson said in a statement, Efe news agency reported.
Despite expressing concern about the plight of the Rohingyas, Washington has declined until now to ascribe their suffering to a deliberate campaign of ethnic cleansing.
The country's "government and security forces must respect the human rights of all persons within its borders, and hold accountable those who fail to do so," the secretary said.
"The United States continues to support a credible, independent investigation to further determine all facts on the ground to aid in these processes of accountability," he added.
More than 600,000 Rohingyas have fled to neighbouring Bangladesh since Myanmar launched a military operation against the mainly Muslim minority group following the deaths of a dozen members of the security forces in August 25 attacks by a group calling itself the Arakan Rohingya Salvation Army.
State Department officials said Wednesday in a conference call with reporters that President Donald Trump's administration is weighing the idea of imposing sanctions on specific individuals in Myanmar found responsible for what they described as "organised and planned" ethnic cleansing.
In a report presented on Tuesday in Bangkok, Amnesty International accused Myanmar of subjecting the Rohingya to a system of "institutional" discrimination tantamount to apartheid.