At least 15 people, mostly children, drowned and scores more were feared missing when a boat carrying Rohingya refugees capsized off Bangladesh Thursday, police said, as the number of people fleeing violence in Myanmar topped half a million.

Witnesses and survivors said the boat overturned just yards from the coast in rough waters, after it was lashed by torrential rain and high winds.

"They drowned before our eyes. Minutes later, the waves washed the bodies to the beach," said Mohammad Sohel, a local shopkeeper.

The tragedy is the latest in a series of deadly accidents as desperate refugees surge across the border into Bangladesh from neighbouring Myanmar, where the country’s military has been accused of ethnic cleansing against Rohingya Muslims.

It came as the United Nations, which has registered more than 500,000 refugees in recent weeks, said a planned visit to Myanmar’s conflict-battered Rakhine state had been postponed Thursday due to bad weather, thwarting efforts to reach the isolated epicentre of the violence.

Local police inspector Moahmmed Kai-Kislu told AFP 15 bodies including at least 10 children and four women had so far been washed ashore, and there were fears the number could rise still further.