The United Nations today drastically increased the estimated number of Rohingya Muslims who have fled violence in Myanmar to 480,000 as Bangladesh eased restrictions on aid groups working in refugee camps and sought $250 million to cope with the crisis.

The number who have crossed the border since August 25 to escape a Myanmar military crackdown has increased by 45,000 in two days.

Between the new arrivals and some 300,000 Rohingya who were already living in the area due to previous violence in Myanmar, there are now nearly 800,000 refugees in camps around the Bangladesh border town of Cox's Bazar that are bursting at the seams.

The situation has forced new arrivals into makeshift shelters in grim conditions, and sparked warnings that epidemics, including cholera, could easily spread.

A report by UN agencies and international charities said the higher number was due largely to an estimated 35,000 Rohingya, not previously accounted for, moving into two refugee camps.

It also said numbers crossing the border had started to rise again.

After reporting a significant fall in arrivals last week, the new report said hundreds had been crossing the border daily in recent days.

'Herculean task'

Pressure on Bangladesh has increased so much that it has eased restrictions on private aid groups so they can work in Cox's Bazar camps.

The country has previously strictly limited access. It has never given reasons, but is sensitive about security and authorities fear a Muslim influx could tempt extremist groups.

The government NGO Affairs Bureau has now cleared 30 local and international groups to meet "emergency needs" in camps and said more would follow, Shahdat Hossain, a bureau director, told AFP.

The aid groups still only have permission to work in the camps for two months and must focus on providing healthcare, sanitation facilities and shelters for the Rohingya, Hossain said.

Authorities had previously only let four international groups -- including Doctors With Border (MSF) and Action Against Hunger (ACF) -- provide food and healthcare.

Muslim Aid, a British-based charity, was given permission to operate in Cox's Bazar last week but this was quickly revoked again. It remains barred even though it operates in other parts of Bangladesh.

Authorities briefly ordered MSF, ACF and Muslim Aid to stop providing aid to the Rohingya in 2012.

Dhaka-based BRAC, one of the world's largest charities, is among the new groups allowed into the camps.

BRAC senior director Asif Saleh said in a Facebook post that the scale of the "humanitarian crisis" was significantly worse than what was being portrayed by the media.

He said the group has taken on the "herculean task" of setting up 15,000 toilets, 1,100 tube wells, 50 health camps, 10 delivery centres and 50 child centres.

Desperate for help

Bangladesh has deployed dozens of emergency medical teams and sent reinforcements to hospitals in Cox's Bazar.

They have treated more than 2,350 Rohingya for serious injuries sustained in the crackdown, including bullet and machete wounds and landmine injuries.

Some 80,000 Rohingya children have also been vaccinated for measles, rubella and polio diseases and thousands of adults treated for diarrhoea, respiratory diseases and pregnancy complications.

Desperately needing more help, junior health minister Zahid Malek said Dhaka has sought $250 million from the World Bank to provide healthcare to the Rohingya.

A United Nations official last week said it would need $200 million over the next six months to handle the Rohingya crisis.

The UN made an emergency appeal for $78 million on September 9, but UN resident coordinator in Bangladesh, Robert Watkins, said much more would be needed as the exodus grows.