Severe flooding left 15 Colorado National Guard members and other emergency workers stranded on high ground near Lyons, Colo., Monday.
The troops and emergency workers were evacuating residents Sunday night, but were forced to abandon their mission when floodwaters began to rise, CNN reported.
They are stuck at their position until either the floods recede or weather clears and a helicopter is able to rescue them.
A U.S. Army helicopter was able to rescue the residents and some of the troops and emergency workers before the weather became impassible.
The 15 spent the night on high ground where they remained Monday, the National Guard said.
Meanwhile, Colorado's presumed flooding death toll rose to six, with more than 1,250 people still missing and about 19,500 homes damaged or destroyed, officials said.
The number of confirmed or presumed dead included an 80-year-old woman and a 60-year-old woman who disappeared in raging floodwaters, a Larimer County Sheriff's Department spokesman in Fort Collins said.
The older woman "was injured and couldn't get out of her home," John Schulz said. "When people came back to help her, the house was gone."
The younger woman's home was also swept away in the same area, police said.
Hundreds of National Guardsmen, sheriff's deputies and firefighters searched for more than 700 people in Boulder and Larimer counties, officials said.
"We have no idea if there are more victims," Boulder County Emergency Management spokesman Andrew Barth said.
More than 550 people in the 13 other flooded counties were also unaccounted for, The Denver Post reported, meaning those people had not yet communicated with family, friends or authorities, the newspaper said.
Early damage tallies by the state Office of Emergency Management indicated about 18,000 homes were damaged and more than 1,500 were destroyed. Nearly 12,000 people were evacuated.
President Barack Obama ordered Federal Emergency Management Agency Administrator Craig Fugate to travel to the Colorado Monday to "ensure the federal government is closely coordinating with the state and local response," the White House said Sunday.
Obama called Colorado Gov. John Hickenlooper to get an update on the situation and express his concern for residents affected by the historic flooding the National Weather Service called "biblical."
Rain and thick clouds grounded emergency aircraft that had been rescuing people, officials said.
"There's a heavy, heavy fog, and rain is coming down hard," the Post quoted Barth as saying. "Standing water is rising because the ground is saturated."
Authorities across the flooded areas warned infrastructure, including new roads, would not be completed for many months.
More than 4,000 customers were without gas in Boulder County alone.
The flooding started a week ago as a slow-moving cold front stalled, clashing with warm humid monsoonal air from the south. The situation intensified Wednesday and Thursday. Boulder County was worst hit, with some 18 inches of rain recorded by Sunday.