A powerful storm raging with winds of up to 175 kph (109 mph) is heading to the coasts of eastern Bangladesh and Myanmar, threatening around a million Rohingya refugees and others living in low-lying locations.
Thousands of people in both countries have already fled to safer areas ahead of the storm, officials said on Saturday.
Cyclone Mocha is likely to intensify further and make landfall on Sunday between Cox's Bazar in Bangladesh and Myanmar, the Bangladesh Meteorological Department said in a bulletin.
Cox's Bazar, a southeastern border district, is where more than a million Rohingya refugees live, most of them having fled a military-led crackdown in Myanmar in 2017. Categorised as a very severe cyclonic storm that could unleash sea surges of up to 12 feet (3.66 metres), Mocha is expected to hit Myanmar's Rakhine state and northwestern region, where six million people need humanitarian assistance and 1.2 million are displaced, the UN humanitarian office said.
Since a junta seized power two years ago, Myanmar has been plunged into chaos and a resistance movement is fighting the military on multiple fronts after a bloody crackdown on protests.
A spokesperson for the Myanmar junta did not respond to a phone call. Focus on 'saving lives'
In Bangladesh, Mohammad Shamsud Douza, a government official responsible for refugees, said: "We are focusing on saving lives ... people who are at risk of landslides will be evacuated."
Thousands of community workers and volunteers had already been deployed, alongside medical and rescue personnel who are on stand-by, he said.
Outside the refugee camps, at least 5,000 people have moved to cyclone shelters and authorities have made arrangements to evacuate 500,000 people from the path of the storm, said Netai Chandra Dey Sarker from Bangladesh's Department of Disaster Management.
In Myanmar, the World Food Programme said it was preparing food and relief supplies that could help more than 400,000 people in Rakhine and surrounding areas for a month.
At least 10,000 have left their homes in Myanmar's Rakhine state for safer areas, local media reported.
"Everyone is trying to leave town since yesterday afternoon," said a 20-year-old resident of Rakhine's capital Sittwe, asking not to be named.
"Not many people remain in my street, just my family."