GENEVA - Life-threatening levels of malnutrition have risen dramatically among Rohingya refugee children who have fled Myanmar for Bangladesh, the United Nations warned Friday.

The UN children 's agency said preliminary data indicated a full 7.5 percent of the children crammed into one of the camps in Bangladesh's Cox's Bazar district were at risk of dying from severe acute malnutrition .

More than 600,000 Rohingya have fled Myanmar's Rakhine state since late August during military operations that the United Nations has described as ethnic cleansing and the world's most acute refugee crisis.

Around half of them are children .

"It's very worrying to see the condition of children who keep arriving," UNICEF spokesman Christophe Boulierac told reporters in Geneva after a recent trip to the camps.

The agency and its partners are already treating more than 2,000 acutely malnourished children at 15 treatment centres, and are in the process of setting up six additional centres.

AFP correspondents on the ground also witnessed scores of obviously malnourished children near treatment centres in several refugee camps in Cox's Bazar.

"He cannot eat enough food as he suffers from pneumonia, fever and diarrhoea," Rohingya woman Fahima Bibi told AFP as she emerged from one centre with her two-year-old grandson Mohammad Jabed, who is suffering from severe acute malnutrition .

UNICEF said its preliminary findings were based on a nutrition assessment conducted last week of children under the age of five in 405 households in the Kutupalong refugee camp in Cox's Bazar.

"The Rohingya children in the camp, who have survived horrors in Myanmar's northern Rakhine State and a dangerous journey here, are already caught up in a catastrophe," UNICEF Bangladesh Representative Edouard Beigbeder said in a statement. "Those with severe malnutrition are now at risk of dying from an entirely preventable and treatable cause," he warned. Malnutrition rates among children in northern Rakhine were already above emergency thresholds before the latest crisis erupted.

"The condition of these children has further deteriorated due to the long journey across the border and the conditions in the camps," the UNICEF statement said.

The journey by boat is particularly treacherous, with the International Organization for Migration on Friday putting the number of drownings since August at around 250.

Some 26,000 people now live in the Kutupalong camp, where they are faced with an acute shortage of food and water, unsanitary conditions and high rates of diarrhoea and respiratory infections, the agency said.

UNICEF said it was planning two additional assessments in other sites in Cox's Bazar this month to help determine if the numbers found in Kutupalong might apply to the entire area.

Boulierac however told AFP the agency was concerned by the preliminary findings, since the Kutupalong camp has existed for a long time and the services there are believed to be better than in the many new, makeshift camps.

The overall rate of life-threatening malnutrition could therefore turn out to be even higher than what was found in Kutupalong, he said.

The influx of refugees is continuing, with the UN refugee agency estimating that some 3,000 people crossed through a single border crossing at Anjuman Para between Wednesday and Thursday alone.

"We need far more attention to the crisis, and far more resources for the response," Beigbeder said, stressing that "these children need help right now."