The number killed by the sudden rise in water levels was uncertain, said judge Rob Kelly of Kerr County, located northwest of San Antonio.

"We're trying to get the identity of these folks," Kelly told a press conference after heavy rain led to the "devastating and deadly flood."

The judge, who oversees disaster relief in the county, said there were "some missing" children who had been attending a summer camp in the valley, which is often the site of flooding.

"But in terms of how many, exactly, how many are missing and unaccounted for, we're not sure about that number," he said.

Texas Governor Greg Abbott said the southern state was "surging all available resources" to the area.

"We didn't know this flood was coming," said Kelly, 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.

According to the National Oceanic and Atmospheric Administration (NOAA), the water level of the Guadalupe rose dramatically overnight, from 7.5 feet (two meters) to 29.5 feet.

The weather service issued a flood warning for Kerr County, urging residents to avoid all travel and 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.