Cyclone Gezani made landfall on Tuesday, slamming into the country's second-largest city, Toamasina, with winds reaching 250 kilometres (155 miles) per hour.

The National Office for Risk and Disaster Management (BNRGC) said it had recorded 20 deaths, many after houses had collapsed.

Fifteen people were missing and at least 33 had been hurt, it said, updating earlier tolls.

"What happened is a disaster: nearly 75 percent of the city of Toamasina was destroyed," said the country's new leader, Colonel Michael Randrianirina, who had travelled to Toamasina ahead of the cyclone's landfall to support residents.

"The current situation exceeds Madagascar's capabilities alone," said Randrianirina, who seized power in October, calling on "international partners and donors" to support the impoverished island.

Drone footage shared by the BNRGC on social media showed major flooding in the east coast city of 400,000 people, about 220 km northeast of the capital Antananarivo, with residents wading through water and roofs ripped off buildings.

The city appeared battered, its streets littered with trees uprooted by the force of the cyclone.

The storm had also caused carnage in the Atsinanana region surrounding the city, the authority said, adding that post-disaster assessments were still underway.

"It's total chaos: 90 percent of house roofs have been blown off, entirely or in part," said the head of disaster management at the Action Against Hunger humanitarian group, Rija Randrianarisoa.

"The roads are completely inaccessible because of trees on the ground, sheet metal," he told AFP.

- 'Monstrous' -

The CMRS cyclone forecaster on France's Reunion island confirmed Tuesday that Toamasina had been "directly hit by the most intense part" of the storm.

The cyclone's landfall was likely one of the most intense recorded in the region during the satellite era, rivalling Geralda in February 1994, it said. That storm left at least 200 dead and affected half a million more.

A Toamasina resident told AFP by telephone late Tuesday that the winds had collapsed solid walls. "It's monstrous," the resident added.

Commercial flights to Toamasina airport were suspended except for humanitarian and military flights, airport management told AFP.

Fifteen members of the army's civil protection unit were dispatched to assist with rescue operations, authorities announced.

The cyclone weakened after landfall but continued to sweep across the island, posing the risk of flooding despite being downgraded to a tropical storm.

It is forecast to return to cyclone status as it reaches the Mozambique Channel, according to the CMRS, and could from Friday evening strike southern Mozambique, which has already faced devastating flooding since the beginning of the year.

Besides Geralda in 1994, Cyclone Gretelle, which killed 152 people and displaced 60,000 in 1997, and Gafilo, which left 241 dead and affected more than 300,000 others, have also been particularly deadly.

More than 70 percent of houses in Madagascar are built from precarious materials such as clay, branches or foliage, according to the national statistical institute.

Cyclone season in the southwest Indian Ocean typically lasts from November to April and sees around a dozen storms each year.