Deadly flash floods have killed at least three people in the US state of Colorado, as residents were evacuated, roads closed and a dam was breached, authorities said Thursday.

Two people were confirmed dead near Boulder, northwest of Denver, while one body was recovered from floodwaters in Colorado Springs, south of the state capital.

"We anticipate that there could be others," Commander Heidi Prentup of the Boulder County Sheriff's office said at a noon (1800 GMT) press briefing on the disaster.

After driving rain lashed much of the northeastern part of the state, the National Weather Service urged residents to "move to higher ground now. Act quickly to protect your life."

The rain eased somewhat Thursday morning, but Boulder City spokeswoman Sarah Huntley warned residents to remain on alert.

"It might seem that the threat is abating. That is not actually the case. The risk is still very real," she told reporters, adding: "There are big walls of water."

Firefighters in Colorado Springs recovered one body due to the flooding, spokeswoman Sunny Smaldino said. They rescued a second person stranded by the flood waters, she said.

Meanwhile residents of the small town of Jamestown, just outside the city of Boulder, were ordered to evacuate. "We have two confirmed" dead, Prentup told reporters, adding that helicopters could not get into the area because of the continuing rain.

Officials said a building had collapsed on one of the victims, but further details were unavailable as rescue workers were just arriving in the town.

Mudslides and rockslides blocked many of the roads with debris, hampering rescue efforts, said Gabrielle Boerkircher, a spokeswoman for Boulder County's emergency management division.

"It's been raining for the last three days now," said Boerkircher, adding that the worst "started early in the morning on September 11 and hasn't stopped yet."

The Larimer County Sheriff's office said on its Twitter feed that the Meadow Lake Dam broke.

"The amount of water from broken Meadow Lake Dam is believed to be relatively small. Trying to determine what impact might be," it posted.

Video footage and photographs showed the region under driving rain as water coursed down roadways and washed away vehicles. Standing water reached above car tires in some places.

The National Weather Service warned that "most flood deaths occur in automobiles," adding "flood waters are usually deeper than they appear. Just one foot of flowing water is powerful enough to sweep vehicles off the road."

A mandatory evacuation order was in place for Jamestown, Boulder County's Boerkircher said, adding that hundreds of people elsewhere in the county had voluntarily gone to shelters, including some 200 people in Lyons and around 400 people at Boulder's University of Colorado campus.

Officials at the university said water had leaked into all the buildings, and their main concern was the library, where damage to the books could be extensive, Boerkircher said.

Schools and county offices were closed, as were many roads, due both to flooding and downed trees or debris.

Much of the rain has fallen over a "burn scar" from a forest fire four years ago, she said, explaining that the area "leads into a lot of our creeks and all the creeks end up in our cities."

Boulder County Sheriff Joe Pelle urged residents to "please stay off the roads today," in a message to Facebook.

"Please leave the roads open for emergency crews," he said.