(CNN) A senior United Nations official investigating Myanmar's ongoing crackdown against the Rohingya says she is increasingly convinced it may amount to genocide.

More than 680,000 Rohingya Muslim refugees have fled Buddhist-majority Myanmar to neighboring Bangladesh since August 2017, bringing with them stories of mass murder and destruction at the hands of the country's military.

"I am becoming more convinced that the crimes committed following 9 October 2016 and 25 August 2017 bear the hallmarks of genocide and call in the strongest terms for accountability," said Yanghee Lee, the Special Rapporteur on human rights in Myanmar.

Lee, who was speaking to the UN's Human Rights Council in Geneva on Monday, also called for a body to be set up at Cox's Bazar in Bangladesh, where most Rohingya have sought refuge, to compile evidence of human rights abuses in Myanmar.

She said its "aim would be to facilitate impartial, fair and independent international criminal proceedings in national or international courts or tribunals."