"We have identified 13 fatalities," Kerr County Sheriff Larry Leitha told a press conference after devastating floods swept through the region northwest of San Antonio, warning more casualties were likely.

Some of the dead were children, Texas Lieutenant Governor Dan Patrick said.

"About 23" girls were unaccounted for from Camp Mystic along the Guadalupe River, which rose 26 feet (eight meters) in 45 minutes overnight, Patrick added.

"That does not mean they've been lost, they could be in a tree, they could be out of communication," he said.

Patrick read out a message from the director of the summer camp, which had some 750 campers over the July 4 long weekend, reporting that it had "sustained catastrophic level of flooding."

"We have no power, water or Wi-Fi," the message said.

State and local officials warned against residents traveling to the area which includes camp grounds dotted along the river, with dozens of roads "impassable."

Videos on social media showed houses and trees swept away by the overnight flash flood caused by heavy overnight rainfall of 12 inches -- one-third of Kerr County's average annual rainfall.

Texas Governor Greg Abbott shared a video on X of a victim being plucked from the top of a tree by a rescuer dangling from a helicopter, as floodwaters raged below.

- 'Another wave' -

"Air rescue missions like this are being done around the clock. We will not stop until everyone is accounted for," he said.

Freeman Martin, director of the state's public safety department, said the flood was a "mass casualty event."

About 500 rescue personnel and 14 helicopters were deployed, with the Texas National Guard sending rescue teams and the US Coast Guard joining efforts.

"The rain has let up, but we know there's another wave coming," Martin warned, saying more rain would be hitting areas around San Antonio and Austin.

"We didn't know this flood was coming," Kerr County judge Rob Kelly said earlier on Friday, adding that the region has "floods all the time."

"This is the most dangerous river valley in the United States," he added, referring to the Guadalupe River that flows through the region.

Forecasters issued a flood warning for Kerr County, urging those living near the Guadalupe River to "move to higher ground."

In mid-June, at least 10 people were killed by flash flooding in San Antonio following torrential rains.