Torrential rains have unleashed deadly flash floods along the Guadalupe River in south-central Texas, killing at least 24 people and leaving 23 girls missing from a riverfront summer camp.

Texas Lieutenant Governor Dan Patrick said he was praying for the 23 girls, who remain missing after their Christian summer camp for girls, Camp Mystic, was hit by flash flooding.

He said authorities were searching for the girls, listed as unaccounted for among more than 700 children at several summer camp sites near the banks of the Guadalupe River when the site was inundated by floodwaters at around 4am, while everyone was asleep.

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“We’re praying for all those missing to be found alive,” Patrick said.

The US National Weather Service declared a flash flood emergency on Friday for parts of Kerr County, located in south-central Texas Hill Country, about 100km northwest of San Antonio, following thunderstorms with heavy downpours that dumped as much as a foot of rain.

Patrick said the area has had about 12 inches (300mm) of rain per hour.

Hadley Hannah, Virginia Hollis, Kellyanne Lytal,Janie Hunt,Renee Smajstrla and Lainey Landry were attending Camp Mystic when it was hit by flash flooding. Credit: Supplied.

Camp Mystic said it had informed the parents of all the missing children.

The camp remains cut-off by flood waters, and has no power or electricity.

The remaining children and staff are waiting to be rescued.

During the media conference, Patrick made a promise to the parents of the missing children from Camp Mystic.

“If they are alive and safe, we will find them and bring them home to you,” he said.

“We will do everything humanly possible 24/7, looking in every tree turning over every rock.”

Dalton Rice, city manager for Kerrville, the county seat, told reporters the extreme flooding struck before dawn with little or no warning, precluding authorities from issuing any evacuation orders.

“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.”

Dozens of boat and air rescue crews are still on the look out for people who were washed away in the flash flood. Credit: AP

Kerr County Sheriff Larry Leitha told reporters that 13 people were found dead from flooding in the area, adding, “I think there will be more when this thing is over.”

Texas Lieutenant Governor Dan Patrick said at an earlier news briefing that six to 10 bodies of adults and children had been found, some in cars washed downstream.

He said the Guadalupe River had risen eight metres in 45 minutes amid heavy showers soaking region.

Search teams were flying 14 helicopters and a dozen drones over the area, in addition to hundreds of emergency personnel on the ground conducting rescues from trees and swift-flowing water.

Personnel from the US Coast Guard and the Federal Emergency Management Agency were activated to assist local authorities in confronting the crisis, officials said.

With Reuters and AP