A devastating tropical cyclone has smashed into the South Pacific island nation of Vanuatu and is believed to have caused widespread destruction, with fears that dozens of people may have died.

Communications were knocked out in much of the region by Cyclone Pam, a maximum category five storm, and aid agencies said it could be several hours before a clear picture emerged.

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The UN office for the co-ordination of humanitarian affairs said on Saturday it was checking an unconfirmed report of 44 dead.

Devastation caused by Cyclone Pam. Photo: UNICEF Pacific/AAP

People spent the night in makeshift shelters, the Red Cross Pacific regional head Aurelia Balpe said, as Pam, packing winds of more than 250 kilometres per hour, slammed into the island.

The storm crossed the main Vanuatu island where more than 65,000 people live and a group of islands further south which are home to 33,000 people.

The Australian Bureau of Meteorology, which has been monitoring the storm, said most parts of Vanuatu had been affected.

The storm reportedly changed course at the last minute, hitting the capital Port Vila harder than expected.

“It’s really bad out there. The wind is howling with a deep roar that just doesn’t let up. Anyone not in shelter now is in mortal danger,” the administrator of the Humans of Vanuatu Facebook page wrote.

“The news reports that a communications tower on Ambrym island has fallen down. There seem to have been fatalities, but of course it’s impossible to confirm just now.”

Power was out in much of Vanuatu and communications were cut, although aid agency Save the Children said Pam “is feared to have caused widespread damage” and it was preparing to mount a humanitarian response.

Red Cross was also assessing what was required in terms of food, medicine and shelter.

The Vanuatu Meteorological Service has forecast “very destructive” hurricane-force winds, torrential rain and flash flooding for most of Saturday.

The size of Cyclone Pam can be seen on this wind chart. Photo: ABC