Cyclone Kenneth has made landfall in northern Mozambique, a country still recovering from another huge storm.
The cyclone has brought 220km/h (140mph) winds and has already killed three people on the island nation of Comoros.
Mozambique's National Institute of Disaster Management (INGC) said 30,000 people had been evacuated from areas likely to be hit.
Last month, Cyclone Idai caused hundreds of deaths in the region.
More than 900 people died when the storm brought devastation to Mozambique, Malawi and Zimbabwe.
At least three million people were left in need of humanitarian assistance.
Read also: Cyclone Idai: Tell the world we are suffering - Mozambique survivors
What is the latest?
Kenneth made landfall on the north coast of Mozambique on Thursday evening, with wind speeds equivalent to a Category 4 hurricane.
There is no previous record of hurricane-force systems ever hitting the region so far north before, BBC Weather reports.
It is expected to weaken as it moves inland.
A "compulsory evacuation" of families is under way and will continue "until we have all people in secure ground", INGC spokesman Paulo Tomas said.
More than 680,000 people are said to be at risk from the cyclone, Mozambican officials said on Wednesday. Flights have already been cancelled and schools closed.
Forecasters warn it is likely to be slow-moving, meaning heavy rain is expected to fall on the area for several days. The UN says over 600mm (23.6 inches) of rain may come - nearly double the 10-day accumulated rainfall that caused flooding in the port city of Beira, further south, during Cyclone Idai.
Forecasters at Meteo-France warned that Kenneth could trigger waves up five metres (16 feet) higher than usual off the country's northeastern shores.
"I was quite preoccupied by the sea... the wind was very strong, and I've never seen anything like it in my 15 years in Pemba," a Portuguese owner of a lodge on Wimby beach, Anabela Moreira, told AFP.
The UN's World Food Programme said it was working on an "emergency preparedness plan" with the Mozambican government and other humanitarian groups.
Source: bbc