"The situation in the camps is so incredibly fragile, especially with regard to shelter, food, water and sanitation, that one small event could lead to an outbreak that may be a tipping point between a crisis and catastrophe," said Robert Onus, an emergency coordinator for the international medical humanitarian organisation Medecins San Frontieres/Doctors Without Borders.

"Hundreds of thousands of refugees are living in an extremely precarious situation, and all the preconditions for a public health disaster are there," Mr Onus said.

Doctors say with little potable water available people are drinking water from paddy fields, puddles or hand-made shallow wells which are often contaminated with excreta.

Diarrhoea-related diseases are widespread.

More than 420,000 Rohingya have arrived in Bangladesh since Myanmar security forces launched what the UN describes as a campaign of ethnic cleansing in Rakhine, in response to insurgent attacks on about 30 police posts and a military base on August 25.

They are squatting in makeshift settlements that have merged two pre-existing camps called Kutupalong and Balukhali, creating one of the world's largest refugee populations, aid agencies say.

The refugee agency UNHCR and agencies such as the International Red Cross and Save the Children have launched appeals for donations to deliver emergency supplies to the camps.

"There are no roads in or out of the settlement, making aid delivery very difficult," said Kate White, another MSF co-ordinator.

"The terrain is hilly and prone to landslides and there is a complete lack of latrines," she said.

"When you walk through the settlement, you have to wade through streams of dirty water and human faeces."

Ms White said MSF clinics are receiving adults every day who are "on the cusp of dying from dehydration."

"That's very rare among adults and signals that a public health emergency is just around the corner," she said.

Newly arrived refugees are reliant on humanitarian aid and prices in local markets are skyrocketing and a lack of roads is compromising access to the most vulnerable people.

"With very little money and chaotic, congested and insufficient food distributions, many Rohingya are only eating one meal of plain rice per day," Ms White said.

Medical facilities are overwhelmed treating thousands of patients, who include those suffering from violence-related injuries and sexual violence.

MSF says comprehensive vaccination campaigns for measles and cholera need to be launched immediately to reduce the outbreak of disease and protect the Rohingya and Bangladeshi populations.

The number of Rohingya who have fled Rakhine, including from earlier departures as long ago as 1978 now tops 800,000, aid agencies say.

Human rights groups say more are on the way.

In Rakhine the ability of aid agencies to deliver emergency supplies to stricken communities has been severely restricted after Myanmar's civilian leader Aung San Suu Kyi accused some agencies of being linked to "terrorist insurgents."

She presented no proof.

Helle Thorning-Schmidt, Save the Children's international chief executive, warned that unless relief agencies are able to resume their operations safely in Rakhine the crisis there will worsen.

Save the Children estimates almost 250,000 children have crossed the border into Bangladesh since late August.

Hundreds of them have arrived alone, having been separated from their families in the chaos of escape or because their family members were killed.

"Children with no one to look after them are the most vulnerable and are at increased risk of exploitation, abuse and trafficking," Ms Thorning-Schmidt said.

"We have been shocked by some of the stories our teams have heard from Rohingya children," she said.

In a speech on Tuesday Ms Suu Kyi refused to condemn the military for its brutality in Rakhine, home to 1.1 million Rohingya, who are denied citizenship and other basic rights, despite that they have lived there for generations.