Tiny Mohammad Sohail cries uncontrollably as he waits to see a doctor -- one of the thousands of Rohingya children at risk of an agonising death from malnutrition even after reaching the safety of refugee camps in Bangladesh. 

His father was killed in the crackdown on Muslims in Buddhist-dominated Myanmar's Rakhine state, forcing his mother Hasana Begum to flee with Mohammad and his brother -- joining some 610,000 other Rohingya who have fled since August. 

The family barely ate on their seven-day trek across hills and through jungles to the Bangladesh border where they arrived two weeks ago, and it has taken its toll. Aged just 21 months, Mohammad's ribs nearly poke through his skin. His hands are just skin and bone. 

"We walked for days through continuous rain, cold and heat. Both my sons suffered from fever and diarrhoea and have since lost appetite," Begum told AFP.

There are at least 50 other malnourished children like him at the Balukhali camp medical unit.

"The condition of many of these children is very critical. Most of their parents don't even understand the extent of the problem," said paramedic Shumi Akhter.

Medical teams are distributing special high-nutrition baby food packs so Rohingya infants can build some muscle. But it is a desperate battle for all.

The UN Children's Fund, UNICEF, estimates that 25,000 children in the overcrowded Rohingya camps are suffering from severe malnutrition that could easily become a major killer.

"The Rohingya children in the camp -- who have survived horrors in Rakhine state and a dangerous journey here -- are already caught up in a catastrophe," said Edouard Beigbeder, the country head of UNICEF.

"Those with severe malnutrition are now at risk of dying from an entirely preventable and treatable cause. These children need help right now," Beigbeder said.

More than half of the huge influx into the refugee camps are children. Some have died there, but the UN said it had no information on whether malnutrition was a cause.

- Selling rations -

For widows like Begum who have no extended family, getting food is a new battle as ration queues last between six and eight hours.

"I can't take them to collect the relief as I cannot carry my sons and the heavy sack," the 23-year-old said.

She cannot leave them in the tarpaulin shanty that has become their home either as there is no one to look after Mohammad and three-year-old Nur Alam.

"Every neighbour is busy with their own problems. Nobody has spare time to babysit," she said.

"But I get panicky there until I get back home because these boys are everything I have left," she said.

A visit to shanties at Balukhali showed that most refugee families survive on a diet of rice and lentils, with occasional vegetables and dried fish.

"Such a diet is not sufficient for toddlers or breastfeeding mothers. In this camp, the number of malnourished babies is already over the emergency margin line," aid worker Fazle Rabbi told AFP.

Charity workers said the situation was exacerbated by refugees selling food to local Bangladeshis to raise cash for household goods and other essentials.

"Everyday we buy a lot of food from the refugees. We pay them cash in exchange for rice, lentil, sugar, salt, cooking oil, milk powder and baby food," a Bangladeshi wholesaler in the nearby town of Ukhiya said.

Refugees who admitted selling food said they needed cash to buy firewood, clothing, and other necessities.

The Rohingyas are not allowed to seek work in Bangladesh.

Refugee Karim Majhi said: "We don't have any choice but to sell food."