Aid workers have long warned of a potential humanitarian disaster if there is a major outbreak at the camps in the Cox's Bazar coastal district. The Rohingya, members of a mostly Muslim minority, fled a brutal military crackdown in Myanmar.

Officials said 339 tests have been conducted among Rohingya in the camps so far.

"We are living in fear about what we are going to do if there is a big outbreak," Rohingya refugee Mohammed Rafiq said by phone.

Bangladesh has seen a spike in infections in recent weeks, with 52,445 confirmed cases and 709 deaths.

"We are all working round the clock to ensure that testing is available to refugees," said Louise Donovan, spokesperson for US refugee agency UNHCR in Cox's Bazar.

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"Those who are identified as COVID-19 positive have adequate facilities in place to care for them, and to ensure contact tracing and isolation of those who may have been exposed."

As many as 60,000 to 90,000 people are jammed into each square kilometre, with families of up to a dozen sharing small shelters.

"It is a ticking time bomb," said Alejandro Agustin Cuyar, Relief International charity's Cox's Bazar program director.

Cuyar said the camps were overcrowded, with shared water sources and communal toilets and washing facilities.

"Once the virus takes hold, it will be incredibly challenging to flatten the curve, so we are gravely concerned the numbers needing treatment will soon be overwhelming."

Reuters