For just under two months Rohingya men, women and children have fled their villages from the persecution of the Myanmar military.

"The situation in northern Rakhine state constitutes ethnic cleansing against the Rohingya," US Secretary of State Rex Tillerson said in a statement on Wednesday.

"The United States will also pursue accountability through US law, including possible targeted sanctions" against those who are responsible for alleged abuses.

Over 600,000 Rohingya Muslims have fled Rakhine since August 25. Refugees have described the systematic burning of villages, mass killings and brutal rapes.

The US has shifted its stance in part to increase pressure on Myanmar's military and civilian leaders, who have shared power for the past two years under an uneasy arrangement following decades of military rule.

"These abuses by some among the Burmese military, security forces, and local vigilantes have caused tremendous suffering and forced hundreds of thousands of men, women, and children to flee their homes in Burma to seek refuge in Bangladesh," Mr Tillerson said.

"No provocation can justify the horrendous atrocities that have ensued."

No provocation can justify the horrendous atrocities that have ensued.

Myanmar's Government is led by Nobel Peace Prize laureate Aung San Suu Kyi. She has faced international criticism for her government's response to what is now being called ethnic cleansing. Many are calling on Suu Kyi to step up and show leadership as the situation rapidly declines.

"It's not a situation that is completely under her authority, but certainly we are counting on her to show leadership and to work through the civilian government with the military to address the crisis," a senior US official told reporters in a conference call.

Mr Tillerson said the United States was analysing whether the events in Myanmar constituted genocide or crimes against humanity, which would violate international law. However, this will take time to assess.

A top UN official in September described the military actions as a textbook case of ethnic cleansing, but the United States has avoided using the term until Wednesday.