Torrential rain grounded rescue helicopters in Colorado for much of Sunday, slowing the search for hundreds of people unaccounted for after massive flooding in the western US state killed six, AFP reports.
Over 1,500 homes have been destroyed and more than 17,000 damaged after days of flooding, the Colorado Office of Emergency Management said, while at least five people have died, and the death toll is likely to rise.
President Barack Obama has declared the flooding a major disaster, unblocking federal resources to help a vast operation which has seen more than 11,000 residents evacuated, as roads and bridges have been swept away.
The number of people unaccounted was put as high as 1,253 by the state emergency management office, although Colorado Governor John Hickenlooper put it at 500.
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.
"A number have been collapsed, and we haven't been in them yet," he told CNN.
"Finding the people who are unaccounted for is one of the highest priorities," said Boulder Sheriff Joe Pelle, in an afternoon update posted by Boulder Office of Emergency Management.
Flash floods have afflicted 15 counties down a 200-mile 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.
Rain began pelting the western state earlier in the 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, "that's going to just really magnify the problems we've had so far," Hickenlooper said, in reference to Boulder County.
Liz Donaghey, a spokeswoman for the Boulder Office of Emergency Management, said poor visibility had grounded US National Guard helicopters deployed to the area to help people get out of danger.
Flights resumed late afternoon, but it was unclear for how long.
The region was bracing for another round of heavy rain Sunday evening, tapering off early Monday, according to weather forecasters.
During the day 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.
Raging floodwaters -- already presumed to have killed at least five people -- apparently claimed the life of a sixth.
The latest casualty is an 80-year-old woman who was injured and couldn't get out of her home, the Denver Post quoted John Schulz, spokesman for the Larimer County sheriff's department, as saying.
"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."
Weld County Commissioner Sean Conway called the widespread flooding an "epic event. .. It is a once in 500 years or 1,000 years situation," he told the Denver Post.
Hickenlooper agreed. "This is a heck of a storm," he said, but added later on Twitter: "We've got a lot of broken roads & bridges, but we don't have any broken spirits."