Severe floods from heavy rains have forced thousands to flee homes in Mozambique and left some stranded on roofs as surging waters swamp settlements, aid workers and witnesses said on Monday.

The floods have directly impacted more than 620 000 people, with over 72 000 houses flooded and widespread damage caused to roads, bridges and health centres, said the International Federation of Red Cross and Red Crescent Societies (IFRC), which is assisting with relief efforts.

"Rains are still expected to continue for the coming days, and the water dams are already at full capacity, so the situation could worsen, placing further people at risk," IFRC Program and Operations Manager Rachel Fowler told Reuters from the capital Maputo.

BOATS AND HELICOPTERS USED TO REACH SURVIVORS

Red Cross volunteers have been using small fishing boats to reach survivors but access is becoming increasingly hard, Fowler added. Neighbouring South Africa has deployed an air force helicopter to help.

Hospital worker Celeste Maria told Reuters she and her family fled their home in Chokwe in the southern province of Gaza after authorities sent out flood warnings last week.

"Our home is now completely submerged ... We left behind neighbours who are now telling us they are sheltering on rooftops as the water continues to rise," the 25-year-old said by phone from a resettlement centre.

Aerial videos showed vast areas of land submerged in water, with only the tops of trees sticking out.

There were no immediate estimates of the number of people killed or injured in the latest flooding.

PRESIDENT CANCELS TRIP, PORT OPERATIONS SLOWED

President Daniel Chapo cancelled his trip to the World Economic Forum in Davos and said on social media late on Sunday that "the absolute priority at this moment is to save lives".

The southern African country has been hit by frequent weather-related disasters that scientists say have been exacerbated by climate change.

Portuguese news agency Lusa quoted the National Director of Water Resources Management Agostinho Vilanculos comparing water levels last week to those in 2000 that killed some 700 people.

A spokesperson for Maputo Port, a commodities export hub, said operations were slower than usual but did not stop.

Petrochemical company Sasol and logistics firm Grindrod, two major foreign businesses operating there, said their operations have not been affected at this stage.

Heavy rains have also affected parts of South Africa, including the northeast where the renowned Kruger National Park reopened on Monday after being closed for several days.