The death toll from a category five tropical storm that has hit islands in the South Pacific could run into the dozens, the UN's relief agency says.

Cyclone Pam battered Vanuatu with winds of up to 270kph (170mph).

#Vanuatu preparing for impact of Tropical #CyclonePam - latest reports & maps: http://t.co/RrdDXVzOQh #TCPam pic.twitter.com/KuifkzXo1R

— ReliefWeb (@reliefweb) March 12, 2015

Pictures on social media showed buildings badly damaged and trees and power lines down.

Authorities on the islands had earlier issued a red alert to residents after the cyclone changed direction and began moving towards populated areas.

Communications have been knocked out over a wide area and aid agencies said on Saturday it could be several hours before a clear picture emerges.

PHOTO: Cyclone Pam carved trail of destruction in remote Pacific nation of Vanuatu: http://t.co/B5i1jvJaSH

— The Associated Press (@AP) March 14, 2015

Although thousands of people spent the night in emergency shelters, many more were forced to ride out the storm in their own homes.

Pam had already caused major damage on other Pacific islands, including Kiribati and the Solomon Islands.

Tuvalu, a group of nine tiny islands north-east of Vanuatu, has also declared a state of emergency after the cyclone caused flash floods there.

"The immediate concern is for a very high death toll but also an enormous amount of destruction and devastation," Sune Gudnitz, regional director for the UN's Office for the Co-ordination for Humanitarian Assistance (UNOCHA), told Reuters news agency from nearby Fiji, which is also expecting to be hit by Pam.

There were unconfirmed reports that 44 people had died in Penama province in the north-east of Vanuata, UNOCHA said in a statement quoted by Reuters.

The Vanautu Meteorological Services (VMS) said it expected torrential rainfall, flash flooding, landslides and storm surges.

All six provinces are under red alert, meaning people are advised to immediately head to shelter.

Vanuatu National Disaster Management Office spokesperson Mishaen Garae Lulu told Radio New Zealand that the government had lost contact with some parts of the northern provinces.

He said the cyclone was expected to be worse than Cyclone Uma, which killed 50 people in 1987.

Located about a quarter of the way from Australia to Hawaii, Vanuatu has a population of 267,000 spread over 65 islands. About 47,000 people live in the capital, Port Vila.

On Friday, Alice Clements, an official with the UN children's agency Unicef, told the BBC that the capital had become a ghost town as people took shelter.

"The winds have intensified and the skies have totally clouded over, you can't see the sea or the hills now. Foliage is thrashing around and the wind and rain has been torrential," she said.

"People are anxious; it's been a very long time since Vanuatu has seen a cyclone this big."

Dozens feared dead after Cyclone Pam batters Vanuatu with winds of up to 270kph (170mph) http://t.co/fdwQQeJN3u pic.twitter.com/dh0tUytR5l

— BBC News (World) (@BBCWorld) March 13, 2015

The Vanuatu country director for Save the Children, Tom Skirrow, told the AFP news agency that he was concerned about families living in shanty town areas.

"Thousands of families are living in makeshift, flimsy houses which will not withstand the immense winds and rain we're expecting. Families need to urgently evacuate to safe buildings or the results could be catastrophic."

Beginning to see extent of damage in Vanuatu from mega-storm Cyclone Pam. @350Pacific #PrayForThePacific #TCPam pic.twitter.com/FRWlv8bAlU

— 350 dot org (@350) March 13, 2015

Meanwhile, category three Cyclone Olwyn has hit the coast of Western Australia with wind gusts of up to 195kph (120mph).

People in the state's coastal region were warned to move to higher ground to escape dangerous flooding.