The confirmed death toll from massive floods in the US state of Colorado has risen to eight, while more than 600 people remain unaccounted for, officials said early Tuesday.

Helicopters resumed search and rescue missions, helped by clearer weather after days of torrential rain that has left over 1,500 homes destroyed and more than 17,000 damaged.

Five teams from the Federal Emergency Management Agency (FEMA) are bolstering hundreds of state and local officers trying to reach hundreds of residents stranded by the floods, centered on Boulder County north of Denver.

"We have a strong opportunity here, with FEMA's help, to come out of this whole situation with a stronger infrastructure," Colorado Governor John Hickenlooper told a press briefing.

On Sunday, heavy rain prevented helicopters from taking off for most of the day as officials put the number of people unaccounted for at more than 1,200 across the western state.

But on Monday there was even blue sky in some areas, allowing some 21 helicopters to take to the air.

"We are hoping to take advantage of the weather today and get those rescue operations complete," spokeswoman Micki Trost of the Colorado Division of Homeland Security and Emergency Management was quoted as saying by the Denver Post.

In all eight people have died, including three in Boulder County, two in El Paso County, two missing presumed dead in Larimer County, and one from an as yet undetermined locality, said the Colorado Office of Emergency management (COEM) in its latest update.

The number of people unaccounted for stood at 658 Monday it said, stressing however that that number was "approximate and changing," as rescue crews gained access to more places, and as stranded people got in touch with authorities to confirm they were alright.

In all 11,700 people have been evacuated, the COEM said. A total of 17,994 residential structures have been damaged, and 1,502 destroyed.

Many of the missing may simply be unable to report their whereabouts, but Hickenlooper warned that the death toll may increase. "There are many, many homes that have been destroyed," he said.

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

Flash floods have afflicted 15 counties down a 200-mile (320-kilometer) north-south section of the Front Range, where the Rocky Mountains meet the Great Plains, the Colorado Office of Emergency Management said.

"We've got a heck of a lot of communities dealing with a heck of a lot of water," Jennifer Finch, a spokeswoman for Weld County northeast of Boulder, told Denver Channel 7 News on Sunday.

On Sunday, traffic on Interstate 25, Colorado's main north-south thoroughfare just east of the mountain range, was brought to a halt by water covering two of the three southbound lanes, according to an AFP correspondent on the road.

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."

Although skies were clearer Monday, weather forecasters warned that scattered storms could still dump up to an inch of rain in less than 30 minutes, according to KUSA-TV.

Drier, warmer weather conditions are due to return to the battered state on Tuesday, lasting through to Thursday when there is a chance of showers through the region, it said.