Aid workers have warned of an impending humanitarian catastrophe in western Burma as authorities attempt to isolate tens of thousands of the displaced ethnic Rohingya minority in camps described by one aid worker as "open air prisons".

Aid has struggled to reach those affected by sectarian unrest in early June, as abusive treatment by Burmese authorities continues. The UN announced on Friday that 10 aid workers in Arakan state had been arrested, five of whom were UN staff. Some have been charged, although the details remain unclear.

Rates of malnutrition among the Muslim Rohingya, who have borne the brunt of emergency measures implemented in the wake of fierce rioting in early June between the minority group and the majority Arakanese, are said to be "alarming". The vast majority of aid workers assisting the Rohingya in Arakan have been either evacuated or forced to flee in recent weeks.

"We are worried that malnutrition rates already have and will continue to rise dramatically; if free and direct humanitarian access accompanied by guaranteed security is not granted with the shortest delay, there's no way they won't rise," said Tarik Kadir of Action Against Hunger.

The group's staff were forced to leave northern Arakan state, where some 800,000 Rohingya live and where malnutrition rates were already far above the global indicator for a health crisis. With scant medical care reaching the area, the situation is likely to worsen.

"There's no way of measuring the impact over the past month because staff have either been evacuated or forced to flee," he said. "And given that rainy season is underway, when you factor in all these other problems, we don't need to measure it to know it's a catastrophe."

President Thein Sein, who has been internationally lauded for spearheading Burma's reform, on Wednesday unsuccessfully requested UN help in resettling abroad nearly one million Rohingya. Critics have likened it to an attempt at mass deportation.

Elaine Pearson, deputy Asia director at Human Rights Watch (HRW), said the group "would expect a strong international response" to any attempt to deport the Rohingya. HRW staff who recently returned from Arakan state said that while both Rohingya and Arakanese were complicit in "terrible violence" during the June rioting, the subsequent mass arrests "focused on Rohingya".

"Local police, the military, and border police have shot and killed Rohingya during sweep operations, [while] those detained are being held incommunicado," she said.

A resident of Maungdaw in northern Arakan said he had witnessed Rohingya men and children as young as 12 being tortured in a police station in early July. After interrogating them about arson attacks in the town, police "handed them over" to Arakanese youths inside the station.

"I saw these youths burning the testicles and penis of old men with a cheroot [Burmese cigar] and also hitting young Muslim detainees with an iron rod and pushing a wooden stick in their anus."

The official death toll of the rioting and its aftermath has been put at 78, although the real figure may be much higher. International observers are banned from visiting northern Arakan state, where the majority of Rohingya live, making accurate data collection impossible.

The violence, which was triggered by the rape and murder of a local Arakanese woman by three Muslim men in late May, has pitted the Rohingya population against the majority Arakanese. A law enacted in 1982 refuses to recognise the Rohingya as Burmese citizens, and hundreds of thousands have fled to Bangladesh. Campaigners have called for the law to be overhauled.

The aid problems coincide with a dramatic rise in food prices in Arakan. Chris Lewa, from the Arakan Project, which monitors abuse of Rohingya, said that a group of monks had reportedly blocked food aid from entering a camp near the state capital Sittwe.

Similar reports have surfaced elsewhere in the state, largely focusing on Rohingya camps. Lewa feared it was an attempt to "starve them out" and force them to flee the country. Kadir warned that the lack of food and medical care meant that "the possibility of actual starvation certainly is there".

The government earlier announced that the camps for Rohingya would remain in place for one year. Kadir said however that this "goes against everything the aid community demands – to let them be independent as soon as possible is critical".

One aid worker, who requested anonymity, said sanitation in the camps is dire, and likened them to "open air prisons".

Kadir said that humanitarian access to victims of the violence "is fundamental". A World Food Programme spokesperson said it was attempting to provide for around 100,000 people affected by the unrest.