The death toll from a storm that struck three southern African countries rose to 77 on Thursday as emergency teams battled to repair damaged infrastructure and help tens of thousands of victims.

Bringing torrential rains, Tropical Storm Ana made landfall Monday in Madagascar before ploughing into Mozambique and Malawi.

Rescue workers and authorities across the three countries were still assessing the full extent of the damage, even as another storm was forming in the Indian Ocean.

Madagascar on Thursday night declared a state of national disaster as the death toll rose to 48.

Mozambique reported 18 killed while 11 had died in Malawi.

Remnants of the storm have passed over Zimbabwe, but no deaths have been reported there.

In the three hardest-hit countries, tens of thousands of homes were damaged. Some collapsed under the heavy rain, trapping victims in the rubble.

Swollen rivers washed away bridges and submerged fields, drowning livestock and destroying the livelihoods of rural families.

In Madagascar, 130,000 people fled their homes. In the capital Antananarivo, schools and gyms were turned into emergency shelters.

"We only brought our most important possessions," Berthine Razafiarisoa, who sheltered in a gym with his family of 10, told AFP.

In northern and central Mozambique, Ana destroyed 10,000 homes and dozens of schools and hospitals, while downing power lines.

Mozambique and international weather services warned that another storm, named Batsirai, has formed over the Indian Ocean and is expected to make landfall in the coming days.

It "might evolve into a severe tropical storm in the next few days," the UN said in a statement.

Up to six tropical cyclones are expected before the rainy season ends in March.

"The situation is of extreme concern" and "vulnerability is very, very high," said UN Resident Coordinator in Mozambique Myrta Kaulard. 

"The challenge is titanic, the challenge is extreme," she said, noting that the storms are hitting "an already extremely vulnerable" region still trying to recover from cyclones Idai and Kenneth that hit the region in 2019. 

"Mozambique is responding to a complex crisis in the north which has caused an additional enormous strain on the budget of the country, on the population," Kaulard said. "In addition there is also Covid."

In neighbouring Malawi, the government declared a state of natural disaster.

Most of the country lost electricity early in the week, after floodwaters hit generating stations. Power was restored by Thursday in parts of the country, but parts of the electric grid were destroyed.

"Our priority now is restoring power to health establishments, water treatment distribution systems and schools," the national power utility said in a statement.

Southern Africa, and especially Mozambique, has suffered destructive storms repeatedly in recent years.