Emergency workers involved in a massive effort to rescue stranded flood victims in Colorado, where more than 500 people are still unaccounted for, are bracing for a fresh pounding from storms.
Officials said efforts to locate those in need of help were hampered by flood damage to many mobile phone towers.
New flash floods were expected to inundate the area, which thousands were forced to evacuate.
Raging floodwaters in the city of Boulder, already confirmed to have killed at least four people, apparently claimed the life of a fifth 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 presumed dead', after floodwaters destroyed her house, and officials warned the toll will likely climb further.
'There might be further loss of life,' Boulder County Sheriff Joe Pelle told reporters on Sunday. '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.'
But some additional help was on the way, with President Barack Obama declaring a major disaster in Colorado and ordering 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.
And the Wyoming National Guard was helping the evacuation effort after Governor Matt Mead activated five UH-60 Black Hawk helicopters and 20 crew members, the state's military department said.
In the disaster zone, helicopters circled above submerged houses in a search for survivors in the western US state, with hundreds still missing.
About 350 people were unaccounted for in Larimer County alone, from where about 475 people were evacuated, according to the sheriff's office.
In neighbouring Boulder County, 231 people were unaccounted for, according to CNN, though authorities cautioned that the numbers were fluctuating.
'It is no doubt an epic event,' Weld County Commissioner Sean Conway told The Denver Post. 'It is a once in 500 years or 1000 years situation.'
Search and rescue teams are being deployed to assess the situation and contact stranded residents.
The US National Guard provided seven helicopters to help get people out of danger.
About 1200 residents were pulled out of the Pinewood Springs area by the National Guard and Fort Carson personnel, state authorities said on Twitter.
But many others are still awaiting rescue, which authorities said could take days for some.
Impassable roads forced authorities to use a helicopter to evacuate 200 residents from Jamestown, northwest of Boulder, according to news reports.
Residents' furry friends 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 tweeted.
Officials said there were widespread power outages as streets became raging rivers after the state received months' worth of rain in just a few days.
Rain began pelting the state earlier this week, in Boulder, which saw 18.3 centimetres of precipitation in about 15 hours beginning from Wednesday night, with more downpours likely over the weekend.
Pictures from helicopter cameras showed heavy rain had reduced the towns of Jamestown, Lyons and Longmont to little more than islands, with ready-to-eat meals being dropped to stranded, anxious residents below.