Some 32 people, including 14 children, have been confirmed dead following flash floods in central Texas, authorities said on Saturday as rescuers continued a frantic search for campers, vacationers and residents who were still missing.

Officials said more than 850 people had been rescued, including some who were clinging to trees, after a sudden storm dumped up to 15 inches of rain in an area around the Guadalupe River, about 85 miles (137 km) northwest of San Antonio.

Among the missing were dozens of people from the Camp Mystic summer camp, most of them reported to be young girls, after river waters rapidly rose 29 feet.

"We know that the rivers rise, but nobody saw this coming," said Kerr County Judge Rob Kelly, the top local official in the region.

Kerr County Sheriff Larry Leitha said eight of the confirmed dead, including three children, had yet to be identified.

The US National Weather Service said the flash flood emergency has largely ended for Kerr County, following thunderstorms that dumped more than a foot of rain. That is half of the total the region sees in a typical year. A flood watch remained in effect until 7 p.m. for the broader region.

Kerr County sits in the Texas Hill Country, a rural area known for rugged terrain, historic towns and tourist attractions.

Texas Lieutenant Governor Dan Patrick said an unknown number of visitors had come to the area for an Independence Day celebration by the river.

“We don't know how many people were in tents on the side, in small trailers by the side, in rented homes by the side," he said on Fox News Live.

Camp Mystic had 700 girls in residence at the time of the flood, according to Patrick. Another girls' camp, Heart O' the Hills, said on its website that co-owner Jane Ragsdale had died in the flood but no campers had been present as it was between sessions.

Texas Governor Greg Abbott said at a news briefing that he had asked President Donald Trump to sign a disaster declaration, which would unlock federal aid for those affected. U.S. Homeland Security Secretary Kristi Noem said Trump would honour that request.

Trump said earlier in the day that he and his wife Melania were praying for the victims. "Our Brave First Responders are on site doing what they do best," he said on social media.

Trump has previously outlined plans to scale back the federal government's role in responding to natural disasters, leaving states to shoulder more of the burden themselves.

Videos posted online showed bare concrete platforms where homes used to stand and piles of rubble along the banks of the river. Rescuers plucked residents from rooftops and trees, sometimes forming human chains to fetch people from the floodwater, local media reported.

Dalton Rice, city manager for Kerrville, the county seat, told reporters on Friday that the extreme flooding struck before dawn with little or no warning, precluding authorities from issuing advance evacuation orders as the Guadalupe River swiftly rose above major flood stage in less than two hours.

State emergency management officials had warned as early as Thursday that west and central Texas faced heavy rains and flash flood threats, citing National Weather Service forecasts ahead of the holiday weekend.

The forecasts, however, "did not predict the amount of rain that we saw," W. Nim Kidd, director of the Texas Division of Emergency Management, told a news conference on Friday night.

The weekend disaster echoes a catastrophic 1987 Guadalupe River flood in which 10 teenagers drowned when trying to leave a church camp, according to the National Weather Service. (Reuters)