Bangladesh has imposed a lockdown in a southern district that hosts more than a million Rohingya in an attempt to prevent the spread of the coronavirus in the crowded refugee camps.

"Entry and exit from Cox's Bazar district is prohibited from now on," Kamal Hossain, chief of the district administration, said in a statement on Wednesday, after the country reported 218 cases of coronavirus and 20 deaths.

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Stringent legal actions are to be taken against those who violate the order, Hossain said, adding that the restriction had been imposed in the "public interest".

More than a million Rohingya have been living in squalid camps in the southeastern Bangladeshi district after they fled persecution in the predominantly Buddhist neighbour, Myanmar.

Nearly 750,000 of them crossed the border following the 2017 military crackdown on the minority group in Myanmar's northern Rakhine state.

Aid agencies fear that an outbreak in the tightly packed Rohingya camps, one of the largest refugee settlements in the world, would be tough because of population density and a lack of medical facilities.

Louise Donovan, the UN High Commissioner for Refugees (UNHCR)'s spokesperson, said the Cox's Bazar district has extremely limited capacity to provide intensive care treatment.

But earlier, the government said efforts were being made to improve hospital beds, isolation and quarantine facilities.

A coronavirus case was reported in the district last month, but the patient, who had returned from Saudi Arabia, twice tested negative and was released from hospital.

Bangladesh imposed a nationwide transport restriction alongside the closure of all non-essential public and private offices, as well as schools, in a bid to slow the spread of the virus.