Bangladesh's army was ordered Wednesday to take a bigger role helping hundreds of thousands of Rohingya who have fled violence in Myanmar, amid warnings it could take six months to register the new refugees.

Troops would be deployed immediately in Cox's Bazar near the border where more than 420,000 Rohingya Muslims have arrived since August 25, said Obaidul Quader, a senior minister and deputy head of the ruling Awami League party.

Soldiers would help build  shelters and toilets for the thousands of refugees still sleeping in the open under pounding monsoon rain, Quader told AFP. 

"The army presence is especially needed on the spot to construct their shelters, which is a very tough task, and ensure sanitation," he said.

The latest order came from Prime Minister Sheikh Hasina, Quader said.

The soldiers would also ensure order and assist with distributing relief, a chaotic process that seen stampedes as donors have hurled food and other staples from moving trucks.

Previously troops had been tasked with transporting foreign relief supplies from the country's port city of Chittagong airport to Cox's Bazar where the overcrowded camps are located.

As the handful of ill-equipped camps rapidly reached capacity, Bangladesh announced it would create a new site capable of housing some 400,000 refugees within 10 days.

Extra water pumps have been installed at some locations, and concrete rings for latrines stockpiled along the roadside.

But there were few signs of major construction work underway, with many refugees complaining they were being ordered to move on without any idea where to go.

- "We don't know where to go" -

"We don't know where would we go. We are poor. We managed to buy the bamboo and tarpaulin with people's help, and now I have to relocate again," said Mujibur Rahman, a 48-year-old Rohingya father of 10.

"I don't know when this moving game will stop."

The government has been trying to herd refugees into designated areas, fearful that nearby cities could be overwhelmed if they are left unchecked. 

"I tried to go to the place where the Bangladeshi government said they set aside land for us. But locals drove us out asking for money to settle us down," a Rohingya community leader, Yusuf Majhi, told AFP.

Local authorities have set up a dozen relief centres and several emergency kitchens to streamline aid distribution.

But efforts by the army to officially register the new arrivals amid the crowded camps has been moving at a glacial pace, said Brigadier General Saidur Rahman.

"We are aiming to finish it within five or six months," said Rahman, who heads the registration drive.

More registration boothes would be erected to complete the mammoth task, he added.

Monsoon downpours are compounding the misery. 

Cox's Bazar has been pounded with 21.4 centimetres (8.4 inches) of rain in the past five days, raising fears of landslides in the unstable, muddy hills on which thousands of refugees were camped.

Hundreds of refugees were forced to abandon their shanties Wednesday in a rubber plantation after heavy rain flooded the area, according to an AFP correspondent at the scene.

"My tent has been flooded in knee-deep water. The children are suffering from the cold," said Nur Mohammad, a 62-year-old Rohingya man who arrived in Bangladesh with 16 members of his family.

Rohingya, who are predominantly Muslim, are reviled by many in Buddhist-majority Myanmar. 

The UN human rights chief has described the systematic attacks against the Rohingya minority by Myanmar's security forces as a "textbook example of ethnic cleansing".