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 recorded 20 deaths, many after houses had collapsed.

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

Drone footage shared by 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 roofs ripped off buildings and trees uprooted.

There had been massive damage in the Atsinanana region around the city, the authority said, adding that post-disaster assessments were still under way.

"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.

The cyclone weakened on landfall but continued to sweep across the island, posing the risk of flooding.

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

The country's new leader Colonel Michael Randrianirina, who seized power in October, was in the city on Wednesday to assess the situation.

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.

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