Cyclone Batsirai swept out of Madagascar on Monday after killing 20 people, displacing 55,000 and devastating the drought-hit island's agricultural heartland, leading the UN to warn of a worsening humanitarian crisis.

Madagascar was already reeling from a tropical storm which killed 55 people late last month, and the latest extreme weather event came as South African President Cyril Ramaphosa said the continent is "bearing both the brunt and the cost" of global warming.

After drenching the fellow Indian Ocean island of La Reunion, Batsirai made landfall in Madagascar's east on Saturday evening bringing heavy rain and winds of 165 kilometres (102 miles) per hour.

Jean Benoit Manhes, a representative of UN children's agency UNICEF in the country, told AFP on Monday that "Batsirai left Madagascar this morning at 7 am (0400 GMT) heading out into the Mozambique Channel."

Madagascar's disaster management agency said that Batsirai had left 20 people dead and forced 55,000 from their homes. 

UNICEF warned that many of the victims were likely to be children, which make up more than 50 percent of the country's population.

City completely destroyed

The cyclone first hit a sparsely populated agricultural area in the country's east on Saturday, before later weakening. The eastern city of Mananjary was "completely destroyed," a resident named Faby said on Sunday. 

In the southern central city of Fianarantsoa, footage showed a building had collapsed into rubble.

As the cyclone moved west inland it caused flooding that ravaged rice fields in the country's central "breadbasket," UNICEF said.

"The impact of the cyclone does not end today, it will last for several months, particularly the impact on agriculture," Manhes said.

"The roofs of several schools and health centres were blown off" in the affected areas, UNICEF said.

Batsirai spared the capital Antanarivo and the island's main port Tamatave, which led to a lower death toll than had been initially feared by the authorities and aid organisations, who had warned that nearly 600,000 people could be affected and 140,000 displaced.

Some 77 percent of Madagascar's 28 million people live below the poverty line and the latest blow comes during a severe drought in the south which has plunged more than a million people into acute malnutrition, some facing famine.

Constant humanitarian crisis 

The cyclone partly destroyed the main road linking the island's north and south, "which will make it difficult to provide access and reinforcements to villages, including in drought-hit areas," Manhes said.

"Madagascar is in a constant humanitarian crisis," he added.

Around 20 roads and 17 bridges have been cut, according to the country's disaster management agency.

Some of the hardest-hit areas such as the city of Manakara had become unreachable, the UN humanitarian agency OCHA said.

A portion of a main road in Ranomafana crumbled while large boulders rolled from adjacent hills onto the tarmac on other sections, an AFP correspondent saw.

The UN on Monday carried out its first flight to try to assess the damage and how best to respond.

President Andry Rajoelina went to Manakara to inspect the damage and distribute relief aid on Monday, according to a post on his Facebook page.

Some 10,000 people on La Reunion were left without electricity on Sunday, three days after Batsirai passed through the French island, injuring 12 people on the way.

Madagascar was still picking up the pieces after Tropical Storm Ana affected at least 131,000 people across the island in late January, with most of the 55 deaths coming in Antananarivo. Ana also hit Malawi, Mozambique and Zimbabwe, causing dozens of deaths.

South Africa's President Ramaphosa told a summit of African leaders on Sunday that the continent was "experiencing the worst impacts of phenomena associated with global warming such as droughts, floods and cyclones". 

"Despite not being responsible for causing climate change, it is Africans who are bearing both the brunt and the cost," he said.