An aerial view of vehicles submerged in floodwaters along the South Platte River near Greenley, Colorado, on Saturday. Farming communities along the South Platte River were ordered to evacuate ahead of a predicted surge in the flooding, which may have claimed a fifth life and has left many still unaccounted for, according to authorities. John Wark / Reuters

Emergency workers in a massive effort to rescue stranded flood victims in Colorado, where more than 500 people are still unaccounted for, braced for a fresh pounding from storms on Sunday.

Officials said efforts to locate those in need of help were hampered by flood damage to cellphone towers.

New flash floods were expected in the area, which thousands were forced to evacuate.

Raging floodwaters in Boulder, already confirmed to have killed at least four people, apparently claimed a fifth life on Saturday - a 60-year-old woman swept away in the torrent.

The Larimer County Sheriff's Office said on Twitter that the woman was "missing and presumed dead", after floodwaters destroyed her house, and officials warned the toll is likely to climb.

"There might be further loss of life," Boulder County Sheriff Joe Pelle said. "It's certainly a high probability. ... With an army of folks and an air show, we're hoping to reach everyone as soon as possible."

Declaring a disaster in Colorado, President Barack Obama ordered federal aid to support state and local efforts.

"Assistance can include grants for temporary housing and home repairs, low-cost loans to cover uninsured property losses, and other programs to help individuals and business owners recover from the effects of the disaster," the White House said.

The Wyoming National Guard was helping to evacuate people, and five UH-60 Black Hawk helicopters and 20 crew members were dispatched.

In the disaster zone, helicopters circled above submerged houses looking for survivors.

About 350 people were unaccounted for in Larimer County alone, from where about 475 people were evacuated, the sheriff's office said.

In Boulder County, 231 people were unaccounted for, CNN reported, though authorities cautioned that the numbers were fluctuating.

"It is a once in 500 years or 1,000 years situation," Weld County Commissioner Sean Conway told The Denver Post.

Search and rescue teams were being deployed to assess the situation and contact stranded residents.

The National Guard provided seven helicopters to help get people out of danger.

Some 1,200 Pinewood Springs area residents were pulled out of the by the National Guard and Fort Carson personnel, state authorities said.

But many others are still awaiting rescue, and authorities said it could take days for some.

Pets and livestock were also stranded by the torrential rains.

"Our victims' advocates told me tonight there were almost as many pets as people getting off the evacuation helicopters today," the Larimer County Sheriff's Office said.

Officials said there were widespread power outages as streets became raging rivers.

Rain began pelting the state earlier this week. Boulder saw 18.3 centimeters of precipitation in a 15-hour period beginning on Wednesday night.

Pictures from helicopter cameras showed heavy rain had reduced the towns of Jamestown, Lyons and Longmont to little more than islands.

Agence France-Presse in Los Angeles