HONG KONG — More than half of the Rohingya refugees who have fled Myanmar in the past three weeks are children, including hundreds who traveled without family members, putting them at particular risk in cramped, muddy camps in Bangladesh, aid workers say.

The United Nations says up to 400,000 Rohingya have fled the state of Rakhine in western Myanmar since Aug. 25 and are now struggling to find food, shelter and clean water in Bangladesh.

“The camps are totally overcrowded,” said Christophe Boulierac, a spokesman for Unicef. “It’s very muddy and raining every day.”

Of those who have made it to Bangladesh since Aug. 25, about half arrived last week, Mr. Boulierac said, placing extreme pressure on the already struggling relief operations.