Rescuers were desperately searching for at least 20 girls missing from a riverside summer camp, after torrential rains caused a "catastrophic" flash flood that killed at least 24 people as it swept through south-central Texas.

"At this point we're at about 24 fatalities," Kerr County Sheriff Larry Leitha told an evening press conference as rescue teams scrambled to locate stranded residents in the region northwest of San Antonio.

Some of the dead were children, Texas Lieutenant Governor Dan Patrick said at a previous news conference.

The county sheriff said there were "kids that are still missing," adding that between 23 and 25 people were unaccounted for.

Lieutenant Governor Patrick previously said "about 23" girls attending a summer camp in the flooded Kerr County were missing.

They were part of a group of around 750 children at Camp Mystic, a girls summer camp along the banks of the Guadalupe River which rose eight metres in 45 minutes with heavy rainfall overnight.

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

Texas Governor Greg Abbott said he was signing a "disaster declaration" to boost resources in counties in the region.

"It's terrible, the floods," US President Donald Trump told reporters Friday night. "It's shocking." (AFP)