Nearly 340,000 Rohingya children are living in squalid conditions in Bangladesh camps where they lack enough food, clean water and health care, the United Nations Children's Fund or UNICEF said on Friday.
Up to 12,000 more children join them every week, fleeing violence or hunger in Myanmar, often still traumatised by atrocities they witnessed, it said in a report "Outcast and Desperate".
In all, almost 600,000 Rohingya refugees have left northern Rakhine state since Aug. 25 when the UN says the Myanmar army began a campaign of "ethnic cleansing" following insurgent attacks.
"This isn't going to be a short-term, it isn't going to end anytime soon," Simon Ingram, the report's author and a UNICEF official, told a news briefing.
"So it is absolutely critical that the borders remain open and that protection for children is given and equally that children born in Bangladesh have their birth registered."
Most Rohingya are stateless in Myanmar and many fled without papers, he said, adding of the newborns in Bangladesh: "Without an identity they have no chance of ever assimilating into any society effectively."
Safe drinking water and toilets are in "desperately short supply" in the chaotic, teeming camps and settlements, Ingram said after spending two weeks in Cox's Bazar, Bangladesh.
"In a sense it's no surprise that they must truly see this place as a hell on earth," he said.
One in five Rohingya children under the age of five is estimated to be acutely malnourished, requiring medical attention, he said.
"There is a very, very severe risk of outbreaks of water-borne diseases, diarrhoea and quite conceivably cholera in the longer-term," he added.
UNICEF is providing clean water and toilets, and has helped vaccinate children against measles and cholera, which can be deadly, he said.
The agency is seeking $76 million under a $434 million UN appeal for Rohingya refugees for six months, but is only 7 percent funded, he said, speaking ahead of a pledging conference in Geneva on Monday.
UN agencies are still demanding access to northern Rakhine, where an unknown number of Rohingya remain despite UN reports that many villages and food stocks have been burned.
"We repeat the call for the need for protection of all children in Rakhine state, this is an absolute fundamental requirement. The atrocities against children and civilians must end," Ingram said.
"We just must keep putting it on the record, we cannot keep silent."