Thousands of Rohingya Muslims continue to flee from Myanmar to Bangladesh and the United Nations is bracing for a possible “further exodus”, the UN humanitarian aid chief said on Friday.

Around 515,000 Rohingya have arrived in Bangladesh from Myanmar’s western state of Rakhine in six weeks since the end of August.

Rights groups say more than half of more than 400 Rohingya villages in the north of Rakhine State have been torched in a campaign by the security forces and Buddhist vigilantes to drive out Muslims, Reuters reported.

Mark Lowcock, head of the United Nations humanitarian office reiterated an appeal for access to the population in northern Rakhine, saying the situation was “unacceptable”.

“Half a million people do not pick up sticks and flee their country on a whim,” Lowcock added, stressing that the scale of the exodus was evidence of a severe crisis in northern Rakhine.

Myanmar has closed most access to the area, but a couple of agencies have offices open there and the International Committee of the Red Cross is helping the Myanmar Red Cross to deliver aid.

“This flow out of Myanmar has not stopped yet, it’s into the hundreds of thousands of Rohingya who are still in Myanmar, we want to be ready in case there is a further exodus,” Lowcock told a news briefing in Geneva.

The UN has “substantial capacity” in Myanmar which can be quickly deployed to northern Rakhine once clearance is granted he added.

A Myanmar official tally says hundreds of people died as violence consumed remote communities, including Rohingya, AFP reported.

There may be up to 100,000 more people in northern Rakhine waiting to cross into Bangladesh, according to the International Organization for Migration.