The United Nations estimates that $200 million will be needed over the next six months to help Rohingya Muslim refugees who have fled to Bangladesh from Myanmar in “massive numbers” to escape a bloody military campaign.

Bangladesh and humanitarian organisations are struggling to help 4,22,000 Rohingya who have arrived since August 25, when attacks by Rohingya militants triggered a Myanmar counter-insurgency offensive that the United Nations has branded ethnic cleansing.

Bangladesh was already home to some 400,000 Rohingya who fled earlier bouts of violence and persecution in Buddhist-majority Myanmar.

The United Nations launched an appeal for $78 million on Sept. 9, but the refugees have kept coming.

“Right now, were looking at $200 million,” Robert D. Watkins, U.N. resident coordinator in Bangladesh told Reuters in an interview in his office in the capital, Dhaka, on Friday.

“It has not been confirmed, but it is a ballpark figure based on the estimates on the information we have,” he said, adding that would be for six months.

“We base these appeals on immediate needs, and right now we know they’re going to be here for six months.”