Cyclone Kenneth, which made landfall in Mozambique on Thursday, has been downgraded after winds weakened, but heavy rainfall is expected for days.
Mozambique was still recovering from Cyclone Idai, which killed more than 900 across three countries in March.
Kenneth hit with winds of 220km/h (140mph), having already killed three people on the island nation of Comoros.
While winds have eased, there are warnings that days of heavy rain could cause serious flooding and landslides.
France’s meteorological agency said 600-800mm of rain was expected to land on Mozambique over coming days.
Mozambique’s emergency situation institute (INGC) said one person had been recorded as killed by Cyclone Kenneth – as a result of a falling coconut tree at Pemba in Cabo Delgado.
On Friday the cyclone was officially downgraded to an ex-cyclone, but aid agencies raised concerns about the effect of the coming rain on communities prone to flooding and landslides.
Mozambique’s National Institute of Disaster Management (INGC) said 30,000 people had been evacuated from affected areas.
What is the latest?
The amount of rain expected – more than 600mm (23.6in) – is nearly double the 10-day accumulated rainfall that caused devastating flooding in the port city of Beira, further south, during Cyclone Idai.
The UN’s World Food Programme said it was working on an “emergency preparedness plan” with the Mozambican government and other humanitarian groups.
Reports from northern Mozambique said many homes had collapsed and the provincial capital of Pemba had lost its power supply.
Kenneth made landfall on Mozambique’s northern province of Cabo Delgado on Thursday evening, with wind speeds equivalent to a category four hurricane.
Communications with the affected areas were disrupted, obscuring the full extent of the damage.
There is no previous record of hurricane-force systems ever hitting the region so far north before, according to BBC Weather analysis.
Thousands remain away from their homes after compulsory evacuation procedures put into place by authorities.
What is the affected area like?
Cabo Delgado province is not as densely populated as the area hit by Cyclone Idai, and there is apparently more high ground there.
That, in addition to warnings by authorities ahead of the storm, could significantly stem losses compared with Cyclone Idai.
But reports said many thousands of homes had been flattened by the winds, and the area has been hit by Islamist violence in recent months, which could complicate humanitarian operations.
Thousands of people had already fled their homes to seek shelter from violence in camps for displaced people.
What about other countries in the region?
Comoros is still reeling from damage caused by the cyclone, which battered the islands with high winds and heavy rains. The winds have caused widespread power outages and damage to homes.
In some southern areas of neighbouring Tanzania, authorities have ordered schools and businesses to close.Residents in the southern Tanzanian town of Mtwara were earlier told to seek higher ground and shelter, but that warning was stood down.
The International Federation of Red Cross and Red Crescent Societies shared images of the damage on social media. In a tweet, the group confirmed it had volunteers on the ground assisting communities.
Despite Zimbabwe being further inland, officials there say they are also putting their disaster management agencies on alert.
“Drawing lessons from Cyclone Idai we cannot take chances any more,” said Department of Civil Protection director Nathan Nkomo.