SEATTLE (Washington) - HEAVY rain pounded the northwest US state of Washington on Thursday, flooding roads, triggering mudslides and leaving millions of residents cut off from the rest of the country.

Melting snow and ice combined with warm torrential rain have combined to leave rivers across the state close to bursting, and forcing 30,000 people to evacuate their homes.

An estimated three million people living along Puget Sound, the coastal inlet where the state's main hub of Seattle is located, were isolated after road and rail links into the area were severed.

The main highway between Seattle and Portland, Oregon, 175 miles to the south, was closed because of flooding 90 miles from Seattle at Chehalis, putting a stop to automobile, truck and bus traffic.

The route is used by 55,000 drivers a day, including 10,000 trucks.

The waters have also stopped all rail traffic, leaving air as the only connection between the cities, authorities said.

Avalanche dangers that accompany the warm rains have closed the only three mountain routes that link Seattle to the eastern half of the state.

State officials estimate the cost of the flooding to commerce at $4 million (S$5.9 million) a day.

'We have totally stopped commerce in the state,' said Ms Paula Hammond, Secretary of the Washington State Department of Transportation.

The road closures have left the shelves of grocery stores in the eastern city of Spokane bare of essentials such as eggs and bread, officials said.

Authorities have also reported several roof collapses in Spokane, which has been buried by two meters of snow since December.

South of Seattle, near the town of Orting, more than 30,000 people have been evacuated from their homes and are now living in Red Cross and other shelters, local emergency management officials said.

Rivers in the usually rainy area are setting records. Nearly two dozen were expected to crest well above flood stage later Thursday or early Friday.

While most of the damage and disruptions have occurred in suburban and rural areas, the city of Seattle was not spared.

Thursday morning saw numerous landslides that blocked streets for morning commuters. Those taking the freeway found massive puddles of standing water slowing their commute.

National Weather Service officials described the flooding as some of the most dramatic in the state's history.

'It's right up there with some of our most memorable flood events,' National Weather Service forecaster Doug McDonnal said on Thursday.

'The thing that's kind of amazing in the past few years is how many flood episodes we've had.'

Forecasters say the rain is expected to ease over the weekend. -- AFP