(Reuters) -- Widespread casualties are feared after a powerful typhoon battered Vanuatu overnight, triggering flash floods and knocking out telephone lines in the isolated Pacific island nation, aid officials said on Saturday.

At least one person has been confirmed dead in Papua New Guinea, one of several other Pacific island nations also affected by the Category 5 storm, named Pam. The United Nations was preparing to deploy emergency rapid response units to Vanuatu on Sunday.

“We fear the worst,” Sune Gudnitz, the regional head of the U.N. Office for the Coordination of Humanitarian Affairs, said, speaking from Fiji.

Gudnitz said the storm's power was comparable to Typhoon Haiyan, which devastated the Philippines in 2013. At present, he said, the only information available from Vanuatu has come from isolated reports on social media, describing a terrifying scene as Pam hit the capital, Port Vila, late on Friday.

UNOCHA said there was an unconfirmed report that 44 people had died in Vanuatu's northeastern Penama province.

“Vanuatu is a very vulnerable place because of its location in the middle of the ocean,” Gudnitz said. “It is possible that there will be a death toll that could be high. I can't give any numbers. I think it is a well-grounded fear.”

The UN will attempt to deploy self-reliant rapid response units to Vanuatu on Sunday, but weather could yet prevent them from landing, he said. Vanuatu has a population of around 260,000 and infrastructure there is poor, especially in the outlying islands.

“We know that traditionally in the Pacific people are not that keen on leaving for evacuation centers,” Gudnitz said.

As the storm bore down on Vanuatu on Friday, aid officials warned that poor construction could put thousands at risk from flash floods and buildings collapsing.

In addition to Papua New Guinea, the storm has affected the Solomon Islands.

An official with the International Organisation for Migration said a team surveying damage in Papua New Guinea's offshore New Britain region said at least one person was known to have died there.

“The floods have destroyed livelihoods and infrastructure. A lot of the damage to houses has come from fallen trees,” said George Gigauri, the IOM's head of mission in Papua New Guinea.

(Reporting by Christopher McCall; Editing by Leslie Adler)