Dhaka stepped up border controls after the latest round of violence began 10 days ago.

A total of 87,000 mostly Rohingya refugees have arrived in Bangladesh since violence erupted in neighbouring Myanmar on August 25, the United Nations said on Monday.

Thousands of the stateless Muslim minority have fled the mainly Buddhist nation and poured over the border since the latest round of fighting broke out, piling pressure on the already overcrowded camps in Bangladesh.

Around 20,000 more were massed on the border between Bangladesh and Myanmar's western state of Rakhine and waiting to enter, the UN said in a report.

Dhaka stepped up border controls after the latest round of violence began 10 days ago.

But in recent days Bangladeshi border guards appeared to be allowing the fleeing refugees to enter and the UN said recent arrivals reported there had been no attempt to prevent them from crossing.

Over the last five years Rakhine has been divided along ethnic and religious lines, but the current violence is the worst yet.

Scores of people have drowned attempting to cross the Naf border river, many in makeshift boats.

Most of the new arrivals have crammed into camps near the border, where the UN said local people were helping the relief effort.

Rakhine has been a crucible of religious violence since 2012, when riots erupted. Scores of Rohingya were killed and tens of thousands of people -- the majority from the Muslim minority -- were forced into displacement camps.

The latest round of violence erupted when Rohingya militants attacked remote police posts, killing 15 officials and burning villages.

Myanmar's army chief has said nearly 400 people have died since then, including 370 Rohingya militants.

Myanmar security forces have launched "clearance" operations to sweep out insurgents whose ranks appear to be swelling as male Rohingya villagers join their cause.

Rights groups allege massacres of Rohingya in remote villages led by Myanmar police and troops and ethnic Rakhine Buddhist mobs.

The Rohingya community numbering roughly one million is reviled in Myanmar. They are seen as illegal immigrants from Bangladesh even though many have lived for generations in Myanmar.