Hundreds of Rohingya rescued by Bangladesh and sent to a flood-prone island after being stranded at sea for weeks should be moved to existing refugee camps, the UN secretary general has said.

The appeal by Antonio Guterres to Bangladesh's foreign minister in a letter obtained Sunday by AFP comes as concern grows over a coronavirus outbreak in the sprawling camps near the border with Myanmar.

A fourth virus case has been detected in the crowded camps, officials said Sunday.

The Rohingya on Bhashan Char island -- a silty strip of land prone to violent and potentially deadly monsoon storms -- were rescued in early May after floating adrift in the Bay of Bengal.

Dhaka has said they were not initially sent on to the camps in the southeast as authorities were afraid they might have coronavirus.

Guterres said the 308 refugees should be moved to the camps after their time in quarantine is completed.

"While those rescued at sea may be quarantined for public health purposes, they must also be extended the protection they deserve as refugees," Guterres said in the letter to Foreign Minister A.K. Abdul Momen dated Friday.

"I trust that they too will benefit from the humanitarian services offered to the Rohingya in Bangladesh and that, at the end of their quarantine period... they will be allowed to rejoin their families in Cox's Bazar."

Dhaka has not said how long the refugees will remain in isolation.

Momen told AFP he had yet to receive the letter, but said those concerned about Bhashan Char -- where facilities for 100,000 people were built last year -- should host the Rohingya in their own countries.

"We don't want any more Rohingya," Momen told AFP.

UN secretary general Antonio Guterres said Rohingya found adrift in the Bay of Bengal should be sent to refugee camps in the country's southeast
UN secretary general Antonio Guterres said Rohingya found adrift in the Bay of Bengal should be sent to refugee camps in the country's southeast AFP / Suzauddin RUBEL

"We don't have any other place to keep them. If they (other countries) don't like Bhashan Char, let them take them back to their countries. (Or) else, let them return to Myanmar."

He said more pressure should be put on Myanmar to repatriate the Rohingya to their home state of Rakhine.

The foreign minister previously said the mainly Muslim refugees would "most likely" have to live in the shelters on Bhashan Char until they return to Rakhine.

An official with the Bangladesh navy, which is looking after the refugees on the island, said they were "comfortable" and were given new clothes and food including iftar -- the daily fast-breaking meal during Ramadan.

He added that authorities were not concerned about a brewing cyclone in the Bay of Bengal as Bhashan Char was fortified with three-metre (9.8-feet) high embankments and had dozens of cyclone shelters.

Nearly one million Rohingya live in squalid camps in Cox's Bazar. Many fled Myanmar after a 2017 military crackdown.

Emergency teams are racing to contain the spread of the coronavirus in the camps. Authorities said Sunday that a fourth Rohingya had tested positive there.

"We have already locked down an entire block, housing 5,500 people," Cox's Bazar health authority chief Mahbubur Rahman told AFP of an area in Kutupalong, the largest of the Rohingya camps.

He said a 200-bed isolation unit was being prepared to treat patients.