Some 600,000 Rohingya children could flee to Bangladesh by the end of the year, a relief group said Sunday, highlighting the scale of the humanitarian crisis triggered by violence in Myanmar's Rakhine state.

More than 400,000 Rohingya Muslims have now arrived in Bangladesh from their Buddhist dominated homeland to escape violence that the United Nations says could be ethnic cleansing.

According to the U.N., more than half of the refugees are children, and more than 1,100 have arrived alone after trekking mud roads and hills for days.

"That number could rise beyond one million by the end of the year if the influx continues, including about 600,000 children, according to U.N. agencies," Mark Pierce, the Bangladesh chief of Save the Children charity, said.

The U.N. has also said it was possible that all the estimated 1.1 million Rohingya could flee Rakhine.

Bangladesh and relief agencies are struggling to cope with new arrivals sheltering on roadsides, hills and open spaces close to existing camps around Cox's Bazar, which borders Myanmar.

Aid agencies have said thousands of Rohingya were half-starving and a major health emergency could break out.

Bangladesh has announced it will build 14,000 shelters for some 400,000 refugees but has said it was also readying a desolate island where many could be relocated.

Pierce said his group was particularly worried about the traumatized children and orphans who have arrived alone in Bangladesh.

"This is a real concern as these children are in an especially vulnerable position, being at increased risk of exploitation and abuse, as well as things like child trafficking," he said.

"Some children have witnessed violence and killing. Some have been shot at, others have seen their homes set on fire. Some have reportedly watched their parents being killed," he said.

The charity said it is setting up safe spaces in the camps for vulnerable children.

They would receive 24-hour support and protection while attempts are made to find family members, it said.

Bangladesh authorities say they are also preparing special measures to care for Rohingya orphans.