Cyclone Idai has triggered a "massive disaster" in southern Africa affecting hundreds of thousands if not millions of people, the UN says.
The region has been hit by widespread flooding and devastation affecting Mozambique, Zimbabwe and Malawi.
Mozambique's President Filipe Nyusi has called it "a humanitarian disaster of great proportion".
He said more than 1,000 people may have been killed after the cyclone hit the country last week.
Cyclone Idai made landfall near the port city of Beira in Sofala province on Thursday with winds of up to 177 km/h (106 mph).
"This is shaping up to be one of the worst weather-related disasters ever to hit the southern hemisphere," Clare Nullis, from the UN's weather agency, told the BBC on Tuesday.
Christian Lindmeier from the UN's World Health Organization, said: "We need all the logistical support that we can get."
Mozambique's government says 84 people have died and 100,000 need to be urgently rescued near Beira.
An aerial survey of the province shows that a 50km (30 mile) stretch of land is under water after the Buzi river burst its banks, charity Save The Children says.
The governor of neighbouring Manica province, Manuel Rodrigues, says there is an urgent need to rescue people still trapped, the BBC's Jose Tembe reports.
"It's very sad and very complicated, given what we saw when we flew over the area. We saw people besieged and asking for help," Mr Rodrigues told reporters.
"They were on top of their roofs made up of zinc sheets. Others under flood waters. We saw many people.
"We can only imagine that they had been there for more than two or three days, without food and without clean drinking water."