US: Colorado floods leave hundreds unaccounted for

More than 500 stranded victims of major flooding in the US state of Colorado braced for a new round of heavy rain today that is threatening to impede rescue efforts.

Officials noted that many of those unaccounted for may simply not be able to telephone loved ones because of flood damage to many cell phone towers.

New flash floods were expected to inundate the area, which thousands were forced to evacuate. A flash flood watch was in effect through the evening for the entire Denver metro area, as well as the northern Front Range Foothills and mountains.

Yesterday, large hail the size of peas or even marbles pummelled parts of the city of Aurora, according to local weather reports. A series of thunderstorms also struck the area.

Raging floodwaters in the city of Boulder, already confirmed to have killed at least four people, apparently claimed the life of a fifth yesterday -- 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 that the toll would likely climb further.

"There might be further loss of life," Boulder County Sheriff Joe Pelle told reporters. "It's certainly a high probability... 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.