At least 31 people have died in Madagascar after a week of heavy rains in the country's northwest, officials said.

At least 15 people remained missing on the popular tourist island, according to the country's national disaster management office, as flooding intensified in the districts of Mitsinjo and Maevatanana.

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The tropical Indian Ocean nation is in the midst of an intense six-month rainy season that often results in casualties and widespread damage.

Strips of road were swept away by the rains and access to affected areas has been cut off.

Torrential rains also made important roads impassable, while a dam near the town of Tanambe burst, flooding surrounding villages and farmland.

'National disaster'

The National Bureau of Disaster Risk Management (BNGRC) warned that flooding in lowland and rice-growing areas also posed a risk of "food insecurity and malnutrition".

A disruption in the supply of basic goods could also lead to a surge in prices, BNGRC added.

The bad weather has displaced nearly 107,000 people, with Prime Minister Christian Ntsay declaring the situation a "national disaster".

"The government is calling on national figures and international partners to help the Malagasy people with emergency aid, early recovery, rehabilitation and reconstruction," spokeswoman Lalatiana Andriatongarivo said in a statement.

The rainy season usually stretches from October to April in the former French colony off Africa's southeastern coast.

Global warming has increased the risk and intensity of flooding, as the atmosphere holds more water and rainfall patterns are disrupted.

Built-up urban areas with poor drainage systems are especially vulnerable to heavy downpours, scientists say.