Wellington, New Zealand — Injured people were arriving at a hospital in Vanuatu as unconfirmed reports of casualties emerged after the South Pacific island nation was rocked by a powerful 7.3-magnitude earthquake that struck just off the coast on Tuesday.

The tsunami warning was canceled less than two hours after the quake. With communications still down hours after the quake and official information scarce, eyewitness accounts of casualties began to emerge on social media and through broken phone calls.

The quake struck at a depth of 35 miles and was centered 18 miles west of Port Vila, the largest city in Vanuatu, a group of 80 islands home to about 330,000 people. The quake was followed by a magnitude 5.5 aftershock near the same location.

It was not immediately clear how much damage had been caused, as phone lines and government websites remained down and official channels were not updated, but reports of widespread destruction filtered through on social media and in interviews.

Dan McGarry, a journalist from Port Vila, told The Associated Press that he heard of one death in the quake from a police officer outside Vila's central hospital. McGarry saw three people on wheelchairs "in obvious distress," he said.

Doctors were working "as quickly as possible" in a triage center outside the emergency room, he added. But the nation is not prepared for a mass casualty event, McGarry said.

Video released by the Vanuatu Broadcasting and Television Corporation showed crowds outside the hospital. Phone numbers for the police, hospital and other community agencies were not returned. There are no official data on casualties.

Reports of people trapped in buildings also cannot be confirmed.

A video posted on social media showed crushed buildings in Port Vila, including one that had collapsed on top of cars. A spokesman for the Fiji Red Cross said the head of the aid agency's office in Vanuatu reported extensive damage before communications were cut.

A four-storey building that houses a number of embassies in Port Vila - including those of the United States, Britain, France and New Zealand - was extensively damaged, New Zealand's foreign affairs ministry said.

A video posted on social media showed the building with some damage, including warped windows and debris that had fallen from the walls to the ground. Other photos and videos show items and shelves that have fallen to store floors and landslides that appear to be blocking some roads.

Agence France-Presse reported that its photographs showed that the ground floor of the building was completely flattened.

The bottom floor "doesn't exist anymore," Vanuatu resident Michael Thompson told AFP by satellite phone after posting photos of the destruction on social media. “It's just completely flat. The top three floors are still holding, but they have fallen.”

But AFP reports that the US Embassy in Papua New Guinea has said all US Embassy staff in Vanuatu are "safe and secure".

"Although the US Embassy building sustained significant damage, all personnel were able to safely evacuate the building," it said in a statement on social media, adding that the embassy would be closed until further notice.

Katie Greenwood, the Fiji-based head of the Red Cross's Asia-Pacific regional office, told the Australian Broadcasting Corp. that downtown Port Vila was filled with large buildings and hotels.

"At the moment we haven't heard of any casualties but I would be shocked if at some point we don't hear this bad news from Port Vila," she said.

McGarry said a "massive landslide" at the international shipping terminal would likely hamper the country's recovery. The airport's runway was also damaged, he said.

Vanuatu's position in a subduction zone - where the Indo-Australian tectonic plate moves beneath the Pacific plate - means that earthquakes above magnitude 6 are not uncommon and the country's buildings are designed to withstand earthquake damage.

"I think it could have been worse," McGarry said. But it was the most serious he had experienced in 21 years in Vanuatu "in the end", he said.

In the hours after the quake, the USGS said the tsunami threat had passed. The agency previously warned of waves up to 3 feet above high tide.

Authorities in Australia and New Zealand, both located in the Pacific Ocean, said there was no tsunami threat to their countries.