Twenty-four people have died in flash floods outside the US city of San Antonio, a county official in central Texas told reporters late Friday, with the toll rising from 13.

"What I can confirm at this point, we're at about 24 fatalities," Kerr County Sheriff Larry Lethia told an evening press conference, later confirming 24 people had died.

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 holiday weekend, reporting that it had "sustained catastrophic level of flooding."

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

Massive rescue efforts were underway to search for the missing girls, with trucks reaching the campsite to start bringing out people on Friday evening, according to US media.

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

Videos on social media showed houses and trees swept away by the flash flood caused by heavy overnight rainfall of up to 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.