Bangladesh has imposed a lockdown on a southern district, home to refugee camps housing more than a million Rohingya Muslims fleeing from Myanmar, to prevent the spread of coronavirus.
Officials said no-one was allowed to enter or exit Cox's Bazar district.
No cases are confirmed in the camps, but aid agencies fear an outbreak could overwhelm poor medical facilities.
Most refugees arrived in the camps following a military crackdown in the neighbouring state in 2017.
Almost 750,000 crossed the border, joining hundreds of thousands of refugees already living there.
Last week Human Rights Watch warned that 350,000 people within Myanmar itself were especially vulnerable to coronavirus.
Cox's Bazar district head Kamal Hossain announced the measures late on Wednesday following a rise in cases in the country as a whole.
The number of infections in Bangladesh has doubled to more than 200 in the last five days, with 20 dead.
"Entry and exit from Cox's Bazar district is prohibited from now on," Mr Hossain said in a statement, adding that "stern action" would be taken against those who violated the order.
Movement of aid workers has been severely restricted.
"Only emergency food supply and medical services can continue work in the camps by maintaining extreme caution," Refugee Commissioner Mahbub Alam Talukder said, quoted by AFP news agency.
Anyone who had arrived in the country recently would have to go into quarantine before visiting the camps, he added.
Aid agencies have expressed fears about the spread of the virus in the camps, because of their cramped living conditions and limited medical provisions including intensive care beds.
Police and soldiers have set up roadblocks on main roads around the district and are patrolling in and around the camps.