Officials suggested many of the victims may simply not be able to call loved ones because of damage to cell phone towers or power outages.

"But we're still bracing," Colorado Governor John Hickenlooper told CNN. "There are many, many homes that have been destroyed. A number have been collapsed and we haven't been in them yet."

Hickenlooper put the number unaccounted for state-wide at 500, but Boulder Sheriff Joe Pelle, in the hardest-hit region, said the numbers were changing rapidly: by mid-afternoon it had been cut to 326, from 431 earlier.

It was unclear if those figures included people still missing in other parts of the state.

"Finding the people who are unaccounted for is one of the highest priorities, and five teams of Boulder County Sheriff's Office detectives are dedicated full-time to this task," said Pelle, in an afternoon update posted by the Boulder Office of Emergency Management.

Thousands have been evacuated after torrential downpours washed away roads and inundated communities, claiming at least six lives.

Brigadier General Peter Byrne and his assigned forces of almost 560 Colorado and Wyoming National Guardsmen have evacuated more than 2,100 people, Pentagon spokesman George Little said.

Active duty forces are also engaged in flood relief efforts under direction of US Northern Command, while seven helicopters from the 4th Infantry Division at Fort Carson have aided search-and-rescue operations.

"This cooperative effort among active and reserve forces to protect lives in Colorado's flood ravaged areas is a testament to the department's ability to provide critical and timely support to FEMA and first responders during a domestic crisis," Little said in a statement.

Rain began pelting the western state earlier this week, with Boulder especially hard hit, seeing 7.2 inches (18.3 centimeters) of precipitation in about 15 hours starting Wednesday night.

And with more downpours affecting already flood-ravaged areas, the situation could get even worse, Hickenlooper said.

With the ground saturated, "that's going to just really magnify the problems we've had so far," he said in reference to Boulder County.

"We're still trying to evacuate people," Hickenlooper said, adding that almost 2,000 residents had been moved out of Boulder alone.

Liz Donaghey, a spokeswoman for the Boulder Office of Emergency Management, said poor visibility had grounded several US National Guard helicopters deployed to the area to help people get out of danger.

"At this point it is a big concern with the weather," she told CNN.

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

Raging floodwaters -- already presumed to have killed at least five people -- apparently claimed the life of a sixth.

The latest casualty was an 80-year-old woman who was injured and couldn't get out of her home, The Denver Post reported.

"There might be further loss of life," Sheriff Pelle told reporters. "It's certainly a high probability... We're hoping to reach everyone as soon as possible."

President Barack Obama has declared a major disaster in Colorado and ordered federal aid to support state and local efforts.

Hickenlooper called the widespread flooding "a heck of a storm."

"We've got a lot of broken roads & bridges, but we don't have any broken spirits," he said on Twitter.