HOUSTON -- At least 13 people were killed and more than 20 children from summer camps along the Guadalupe River went missing during the major flash flooding caused by heavy rain overnight in central Texas, authorities updated Friday afternoon.

There are currently "about 23" kids unaccounted for at Camp Mystic, a private Christian summer camp for girls, Texas Lieutenant Governor Dan Patrick told a news conference Friday afternoon. The camp hosts about 750 children.

He said at least 14 helicopters, 12 drones and more than 500 people are searching around the camp, and multiple adults and children have been rescued from trees nearby.

Kerr County Sheriff Larry Leitha told a news conference that at least 13 people died in the county due to the severe flooding and several people remain unaccounted for.

Leitha said he expects more fatalities will be reported in the county.

"We are still actively trying to find those that are out and those that are needing assistance," Kerrville City Manager Dalton Rice said.

At about 4 a.m. local time (0900 GMT), the National Weather Service upgraded its flash flood warning, which allows wireless emergency alerts to be sent to cell phones for residents and campers along the river, NBC News reported.

The service also warned that a "large and deadly flood wave" was moving down the Guadalupe River.

The Guadalupe River in Kerr County rose from 7.5 feet (about 2.3 meters) to nearly 30 feet overnight and is expected to crest at 34 feet in Spring Branch on Friday afternoon, said the service.

Cars, campers and mobile homes were swept away as the section of the Guadalupe River surged in the county, online videos showed.

Texas Governor Greg Abbott said on X that the state is "surging all available resources" to respond to the flooding. "The immediate priority is saving lives," the governor said.

Over half a million people in central Texas were under flash flood warnings as of Friday afternoon.