The ground floor of a building housing the US and French embassies had been crushed under higher floors, resident Michael Thompson told AFP by satellite phone after posting images of the destruction on social media.
"There's people in the buildings in town. There were bodies there when we walked past," Thompson said.
A landslide on one road had covered a bus, he said, "so there's obviously some deaths there".
The quake also collapsed at least two bridges, according to Thompson, who runs a zipline business in Vanuatu.
The ground floor of a concrete building housing diplomatic missions had been flattened, he said.
"That no longer exists. It is just a completely flat. The top three floors are still holding but they have dropped."
Thompson said the ground floor had been rented out by the US embassy. This could not be immediately confirmed.
"If there was anyone in there at the time, then they're gone," he said.
Most mobile networks had been cut off, Thompson said.
"They're just cracking on with a rescue operation. The support we need from overseas is medical evacuation and skilled rescue, kind of people that can operate in earthquakes."