A powerful storm in the Pacific Ocean was expected to graze Hawaii on Tuesday, delivering strong winds, heavy rain and high surf, US weather forecasters said.

The National Weather Service said while a tropical storm warning and hurricane watch remained in effect for Hawaii's big island, Hurricane Flossie appeared to be slowing down on Monday evening.

Forecast to pass just south of the island on Tuesday, the storm was expected to produce winds of about 40 miles (64 kilometers) an hour, heavy rain of up to 10 inches (25 centimeters) and high surf of up to 15 to 20 feet (four to six meters)on south-facing shores of the big island, meteorologists said.

The storm was moving on Monday nigth in a westerly direction about 260 miles (418 kilometers) south of Hilo on the southern end of Hawaii's big island.

"Flossie is showing definite signs of weakening," the National Weather Service's Central Pacific Hurricane Center said late Monday.

"Aircraft reconnaissance and satellite data have shown a clear weakening trend for Hurricane Flossie," it said.

The storm was classified as a category three hurricane on the Saffir-Simpson scale with maximum winds of about 115 miles an hour but the intensity of the storm would likely decline further as it neared Hawaii, forecasters said.

As state authorities urged residents to prepare for the worst, an earthquake hit the big island on Monday. The 5.4 magnitude quake struck at 7:38 pm local time (0538 GMT), about 25 miles (40 kilometers) south of Hilo, the US Geological Survey said.

There were no reports of injuries or major damage, US media reported.

Governor Linda Lingle on Monday issued a proclamation allowing National Guard units to be activated for emergency assistance and releasing two million doallars in state funds for disaster relief.

Public schools and libraries were closed and state and federal parks on the big island were shut down as a precaution, local media reported.