Haiti started burying some of its dead in mass graves in the wake of Hurricane Matthew, a government official said on Sunday, as cholera spread in the devastated southwest and the death toll from the storm rose to 1,000 people.

The powerful hurricane, the fiercest Caribbean storm in nearly a decade, slammed into Haiti on Tuesday with 145 mile-per-hour (233 kph) winds and torrential rains that left 1.4 million people in need of humanitarian assistance, the U.N. Office for the Coordination of Humanitarian Affairs said.

A Reuters tally of numbers from local officials showed that 1,000 people were killed by the storm in Haiti, which has a population of about 10 million and is the poorest country in the Americas.

The official death toll from the central civil protection agency is 336, a slower count because officials must visit each village to confirm the numbers.

Authorities had to start burying the dead in mass graves in Jeremie because the bodies were starting to decompose, said Kedner Frenel, the most senior central government official in the Grand'Anse region on Haiti's western peninsula.

Frenel said 522 people were killed in Grand'Anse alone. A tally of deaths reported by mayors from 15 of 18 municipalities in Sud Department on the south side of the peninsula showed 386 people there. In the rest of the country, 92 people were killed, the same tally showed.

Frenel said there was great concern about cholera spreading, and that authorities were focused on getting water, food and medication to the thousands of people living in shelters.

Cholera causes severe diarrhoea and can kill within hours if untreated. It is spread through contaminated water and has a short incubation period, which leads to rapid outbreaks.

Government teams fanned out across the hard-hit southwestern tip of the country over the weekend to repair treatment centres and reach the epicentre of one outbreak.