Cars and people were swept out to sea by the fast-churning water as survivors fled to high ground, where they remained huddled hours later.
Signs of devastation were everywhere, with a giant boat getting washed ashore and coming to rest on the edge of a highway and floodwaters swallowing up cars and homes.
Speaking to US network ABC, local Doctor Lemalu Fiu said he believed around 50 were injured, in addition to the deaths.
Hampered by power and communications outages, officials struggled to assess the casualties and damage, but the death toll seemed sure to rise.
The quake, with a magnitude between 8.0 and 8.3, struck around dawn about 20 miles (32 kilometres) below the ocean floor, 120 miles (190 kilometres) from American Samoa, a US territory that is home to 65 thousand people.
The territory is home to a US National Park that appeared to be especially hard-hit.
Holly Bundock, spokeswoman for the National Park Service's Pacific West Region in Oakland, California, said the superintendent of the park and other staffers had been able to locate only a fifth of the park's 13 to 15 employees and 30 to 50 volunteers.
"The centre and our offices are completely destroyed" she said.
The National Park's superintendent was quoted as saying four tsunami waves 15 to 20 feet (4 1/2 to 6 meters) high roared ashore soon afterward, reaching up to a mile (1.6 kilometres) inland.
Residents in both Samoa and American Samoa reported being shaken awake by the quake, which lasted two to three minutes.
"It's really happening and everyone is panicking" he said adding, "the roads are blocked, I mean it's cut into half. Some of the parts of the road are in the ocean, some cars in the ocean too, houses, people," said a student at American Samoa Community College in Pago Pago.
In Pago Pago on American Samoa, amateur video showed the downtown area flooded with debris floating in the high water.
The swamped city streets were littered with partly submerged cars.
A ship was even blown out of Malaloa Marina and swept as far as the island's main highway.
Eni Faleomavaega, who represents American Samoa as a non-voting delegate in the US House, said he had talked to people by telephone who said that Pago Pago, just a few feet above sea level, was levelled.
Several hundred people had their homes destroyed, although getting more concrete information has been difficult, he said.
The initial quake was followed by at least three aftershocks of at least 5.6 magnitude.
Experts said many of the victims of the tsunami would have had precious little time to escape.
"Problem is, where the coast, where the land is, right next to the earthquake, you typically don't have enough time to digest the data and get a warning out in an effective way" US Geological Survey Seismologist Susan Hough said on Tuesday.
The Pacific Tsunami Warning Centre issued a general alert from American Samoa to New Zealand; Tonga suffered some coastal damage from 13-foot (4-metre) waves.
Japan's Meteorological Agency also issued a tsunami warning all along that country's eastern coast.
A meteorologist for the National Weather Service in American Samoa, said at least 19 people were killed in four different villages on the main island of Tutuila.
In neighbouring Samoa, at least three villages were devastated.
An unspecified number of fatalities and injuries were reported in the Samoan village of Talamoa.
The US Federal Emergency Management Agency said it was deploying teams to American Samoa to provide support and assess damage.
The ramifications of the tsunami could be felt thousands of miles (kilometres) away, with federal officials saying strong currents and dangerous waves were forecast from California to Washington state.
No major flooding was expected, however.