Tropical Cyclone Amos continued to intensify on Saturday, meteorologists said, and was likely to near Samoa late on Saturday as a severe category four system.
Amos was upgraded to a category three on Friday with sustained winds of 148km/h at its centre, which were likely to increase to as high as 166km/h in the next 12 to 24 hours.
On Saturday morning, the cyclone was still more than 350km northwest of Samoa's largest island, Savai'i, and was slowly moving southeast at 11km/h while intensifying.
The public in Samoa has been warned to be alert and prepare for possible flooding, landslides and wind damage, the Samoa Meteorological Service said. Heavy rain, strong wind, and flood advisories had been issued for the entire country.
Current forecasts predict the cyclone, with a radius of more than 100km from the storm's eye, will pass close to Samoa's southern coast at about midday on Sunday.
Mulipola Ausetalia Titamaea, a forecaster at the Samoa Meteorological Service, said the country's cyclone centre had been activated and authorities had been briefed as the situation looks to be serious.
"The actual intense wind field is the northern side [of the cyclone], and as the eye moves south of Samoa, we will be bearing the brunt of the system as it tracks on this southeast path," Mulipola said in an interview. "It will impact the whole country."
"People were rushing to the shops in the last few days looking for generators, things to tie down their houses and buying food. It's an indication that people are taking this seriously."
After passing Samoa, Amos is expected to veer south into Tonga's northern islands and weakening, although this system has followed an enigmatic path with varying forecasts, and it could still continue east towards American Samoa.
The territory had been placed under a hurricane watch and a high surf warning had already been issued, the United States National Weather Service in Pago Pago said.
All public schools and some private schools were closed on Friday (Saturday Samoa, NZ time) and authorities had told locals to prepare for the cyclone's arrival.
"It's supposed to start this afternoon," said Fili Sagapolutele, RNZ International's correspondent in Pago Pago, the territory's capital. "We are looking at surfs of 14 to 16 feet this afternoon and 18 to 22 feet tomorrow night and into Sunday morning.
"[Authorities] are asking people who live along the shoreline to be mindful of this situation and to prepare to move as soon as those high surfs start hitting the territory," said Ms Sagapolutele.
Stores in Pago Pago had reported increased sales of plywood, torches, batteries, water and canned food as locals heeded authorities' warnings to prepare and to be ready to evacuate.
Cyclone Amos is approaching the Samoas after passing about 90 kilometres north of the island of Wallis, a French Pacific territory, overnight on Friday.
The territory had been put under curfew overnight as the island braced for winds in excess of 150 km/h, but the cyclone passed further north of the island than feared, and on Saturday Wallis woke to only minor damage, the territory's public broadcaster said.