The toll from a horrifying “wall of death” that swept through central Texas has risen to 32 — including 14 children — with dozens of young girls still missing from a Christian summer camp.

As rescuers continue a frantic search, family members are desperately trawling the Guadalupe River’s banks for loved ones, calling out their children’s names.

The sheriff’s office in Kerr County, the flood’s epicentre, said 800 people had been rescued in the region as flood waters receded.

Harrowing stories have emerged of people clinging to trees and being washed away as gushing floodwaters caught communities by surprise early Friday (local time).

The flash flooding, following thunderstorms that dumped as much as a foot of rain, swept through the Camp Mystic summer camp and smashed into huts and buildings.

The river waters rose almost nine metres rapidly near the camp.

At least 23 to 25 people from the camp remain missing, most of them reported to be young girls. There were 750 children there when the water hit.

Searching for survivors along the Guadalupe River in Kerr County. Photo: AAP

Kerrville city manager Dalton Rice said the extreme flooding struck before dawn with little or no warning.

Authorities were unable to issue advance evacuation orders as the Guadalupe River swiftly rose above major flood level, smashing cars, homes and buildings.

It was reported that the river rose as much as eight metres in just 45 minutes.

“This happened very quickly, over a very short period of time that could not be predicted, even with radar,” Rice said.

“This happened within less than a two-hour span.”

Roads washed away in the sudden wall of water. Photo: AAP

Matthew Stone, 44, of Kerrville, told NBC police knocked on his door at 5:30 am, but he received no warning on his phone.

“We got no emergency alert. There was nothing,” Stone said. Then: “a pitch-black wall of death.”

Erin Burgess told the network she woke to thunder and rain at 3:30am, and 20 minutes later, water was pouring into her home.

She told NBC she clung to a tree for an hour, waiting for the water to recede.

“My son and I floated to a tree where we hung onto it, and my boyfriend and my dog floated away. He was lost for a while, but we found them,” she said.

The US National Weather Service said the flood emergency had largely ended for Kerr County.

A flood watch, however, remains in effect until 7pm on Saturday (local time) from the San Antonio-Austin, Texas, region.

Scattered showers were expected throughout the day, according to Allison Santorelli, a meteorologist with the NWS Weather Prediction Center in College Park, Maryland.

US President Donald Trump said the federal government was working with state and local officials to respond to the flooding.

“Melania and I are praying for all of the families impacted by this horrible tragedy. Our Brave First Responders are on site doing what they do best,” he said on social media.

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

The weather 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 flood almost 40 years ago along the Guadalupe River where a bus and a van leaving a church camp encountered flood waters and 10 teenagers drowned trying to escape, according to a National Weather Service event summary of the 1987 storm.

Hundreds of people were rescued, it said.

-with AAP