The death toll from flooding in the US state of West Virginia rose to 24, as President Barack Obama declared a major disaster, releasing federal aid for the hardest-hit areas.

Days of heavy rain, especially on Thursday and Friday, caused massive flooding in the state, where high waters have washed away cars, trapped hundreds and cut power to large areas.

The president "ordered federal aid to supplement state and local recovery efforts in the area affected by severe storms, flooding, landslides and mudslides," the White House said in a statement on Saturday.

The latest fatality from the flooding was reported on Saturday in Greenbrier County, bringing the death toll to 24, said state spokeswoman Jessica Tice.

More than 21,300 people remained without power across the state late Saturday, according to the West Virginia Division of Homeland Security and Emergency Management.

Hundreds of homes had been severely damaged or destroyed and more than 60 roads remained closed, it said.

Photos on the division's Facebook page showed a muddy, swollen river in the West Virginia town of Clendenin that had spilled over its banks and inundated a nearby neighborhood with floodwater.

The federal support "will provide much needed assistance to severely-impacted regions," West Virginia Governor Earl Ray Tomblin said in a statement.

"As emergency response efforts continue, with members of the National Guard and local emergency responders hard at work helping our neighbors, we will continue pursuing additional assistance for all affected areas."

Federal Emergency Management Agency teams will work with state officials to evaluate the damage in the coming days, and there may be additional requests for federal assistance, Tomblin said.

Sixteen of the victims were killed in Greenbrier, the most mountainous and rugged of the hardest-hit counties, local media reported.

Among the dead was an eight-year-old boy who was swept away while walking along a creek bank with his mother and sister, local news station WSAZ reported. His body was found after three hours of searching.

Cold front ahead

National Weather Service meteorologist Mike Kistner described the downpour to the Charleston Gazette-Mail as "a one-in-500-year event."

While the skies are forecast to be clear on Sunday, cold fronts on Monday and Tuesday are expected to move through the region, both with "high moisture count," the NWS said in a Hazardous Weather Outlook bulletin.

"There is a risk for additional flooding ... especially in the areas hard hit this last week."

Volunteer centers were being set up in five of the worst-hit counties, and Red Cross and community-based shelters had also been opened across the state, the emergency management agency said.

"We have so many businesses, individuals and churches really rallying together and gathering supplies," Tice said.

The flooding forced the PGA Tour to cancel its Greenbrier Classic golf tournament scheduled for July 7-10 in the town of White Sulphur Springs.