(NBC) -- Rescue crews in Colorado, mounting what was called the largest civilian helicopter mission since Hurricane Katrina, were en route to hard-hit towns Monday afternoon in hope of reaching hundreds of people trapped by catastrophic floods.
Authorities dispatched helicopters to rescue an unknown number of people stranded in Jamestown, a northwest Boulder County town of fewer than 300 residents that was left virtually unreachable after floodwaters swallowed huge swaths of the primary access road, said Nick Grossman, a spokesman for the county's office of emergency management.
Jamestown is "essentially an island right now," Grossman said, adding that 45 people have already been evacuated from the flood-ravaged town.
At a news briefing Monday, Gov. John Hickenlooper said saving lives was the state's top priority, adding that 21 helicopters were conducting search-and-rescue operations while officials of the Federal Emergency Management Agency scrambled to help flood-hit families and individuals.
The floods, which Sen. Mark Udall on Monday described as "liquid tornados," have killed at least five people, wrecked more than 17,000 homes and left more than 1,200 people unaccounted for. Phone service, both cell and landline, was out across much of the disaster zone, blocking authorities from knowing the full human toll.
"I'm hopeful that the vast majority of these people are safe and sound," Hickenlooper said on NBC's TODAY. "But we do not have any illusions that there could well be more casualties."
In Larimer County, along the Wyoming line, 16 helicopters from the National Guard and other agencies stood by. Evacuation helicopters were en route just after 1 p.m. (3 p.m. ET) to perform rescue operations for about 1,000 people who remain stranded, the county sheriff's office said.
No deaths have been confirmed in the county, but the sheriff's said two people were missing and presumed dead. At least 396 other people remained unaccounted for, it said.
Damage in some areas of the county was catastrophic: 1,500 homes and 200 businesses destroyed, with 4,500 more homes and 500 other businesses damaged. At least three earthen dams failed.
In Boulder County, closer to Denver, crews were going house to house looking for stranded people. The Army and the National Guard had rescued at least 1,750 people cut off by washed-out roads in the mountain canyons, an Army spokesman said.
Search-and-rescue teams from Utah, Nebraska, Nevada, California and Missouri were in Colorado or on their way to help, according to NBC station KUSA of Denver.
Forecasters said the rain would probably taper off Monday, but the weather presented another challenge — fog.
On Sunday, authorities added 12 counties to a presidential disaster declaration, increasing the total to 15. Federal officials provided food, water, cots and generators, bolstering the state and local response.
People in the 15 counties can also apply for help from FEMA. Those counties cover 4 million people.
Separately, President Barack Obama on Saturday declared a major disaster for Boulder County. That step makes it easier for flood victims to get help for temporary housing and low-cost loans to cover uninsured property losses.