The death toll from catastrophic flooding in the US state of Texas has risen to at least 27, including nine children, local authorities said on Saturday.

"So far, we've evacuated over 850 uninjured people, eight injured people and have recovered 27 deceased fatalities at this time. Of these 27, 18 are adults, nine are children," said Kerr Country Sheriff Larry Leitha.

The flooding began late Thursday and continued into the early hours of Friday, as heavy rains engulfed the Guadalupe River, pushing it to a crest of more than 11.8 metres, surpassing the region's second-largest modern flood in 1987.

Texas Lieutenant Governor Dan Patrick said earlier that about 20 people were still unaccounted for, many from Camp Mystic, an all-girls summer camp.

Describing it as a "catastrophic level" of flooding, Patrick confirmed that "hundreds of people," along with at least 14 helicopters and 12 drones, were on the ground conducting search and rescue operations.

The disaster declaration, signed by County Judge Rob Kelly, stated the flooding caused "widespread and severe damage, injury and loss of life," and there remains "an imminent threat of doing more of the same."

The county's Fourth of July celebrations were cancelled, as floodwaters submerged parks and forced evacuations of households along the river.

Residents were urged to shelter in place and move to higher ground if they were near creeks or streams.

Families plead

Dozens of families shared in local Facebook groups that they received devastating phone calls from safety officials informing them that their daughters had not yet been located among the washed-away camp cabins and downed trees.

Camp Mystic said in an email to parents that if they have not been contacted directly, their child is accounted for.

The camp sits on a strip known as "flash flood alley," said Austin Dickson, CEO of the Community Foundation of the Texas Hill Country, a charitable endowment that is collecting donations to help nonprofits responding to the disaster.

"When it rains, water doesn't soak into the soil," Dickson said. "It rushes down the hill."

Camp leaders said they are without power, Wi-Fi and running water, and the highway leading to the camp has washed away.

Two other camps on the river, Camp Waldemar and Camp La Junta, said in Instagram posts that all campers and staff there were safe.