"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 26 feet (eight meters) 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."

- Rescue efforts -

A massive rescue operation was underway in the region, with around 500 personnel and 14 helicopters helping in the search for survivors.

Texas military official Major General Thomas Suelzer told reporters at least 237 people had been rescued or evacuated by emergency personnel, with 167 rescues performed using helicopters.

Freeman Martin, director of the state's public safety department, told the evening conference: "We had a hard time getting in this morning with the weather the way it was."

"As the day went on, it picked up and we were able to rescue more and recover more, that will continue tomorrow," Martin said, calling the disaster a "mass casualty event."

US media reported that trucks had arrived at Camp Mystic to transport stranded people.

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 flash flood caused by heavy overnight rain of up to 12 inches -- one-third of Kerr County's average annual rainfall.

Governor 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.

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

The Texas National Guard sent rescue teams and the US Coast Guard joined efforts as well.

- 'Another wave' -

Public safety official Martin warned of "another wave" that could impact other counties in the state, adding that "this is not going to end today."

Kerr County officials have repeatedly said they were unaware of an impending flood overnight from Thursday to Friday.

"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.

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

Flash floods, which occur when the ground is unable to absorb torrential rainfall, are not unusual.

But scientists say in recent years human-driven climate change has made extreme weather events like floods, droughts and heatwaves more frequent and intense.

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

In the northeastern state of New Jersey, at least two people were killed when a tree fell on their vehicle during a "severe storm," local police confirmed on Friday.