Torrential rains unleashed flash floods along the Guadalupe River in Texas on Friday, killing at least 24 people as rescue teams scrambled to save dozens of victims trapped by high water or reported missing in the disaster, local officials said.

Among the missing were 23 to 25 people listed as unaccounted for at an all-girls Christian summer camp located on the banks of the rain-engorged Guadalupe, authorities said.

At a news conference late on Friday, almost 18 hours after the July Fourth crisis began, Texas Governor Greg Abbott said search-and-rescue operations would press on through the night and into Saturday.

Abbott said resources devoted to the effort would be "limitless."

Speaking to reporters aboard Air Force One at the end of a day of public events, President Donald Trump said "we'll take care of them," when asked about federal aid for the disaster.

The U.S. National Weather Service declared a flash flood emergency for parts of Kerr County in south-central Texas Hill Country, about 65 miles (105 km) northwest of San Antonio, following thunderstorms that dumped as much as a foot of rain.

Dalton Rice, city manager for Kerrville, the county seat, told reporters the extreme flooding struck before dawn with little or no warning, precluding authorities from issuing advance evacuation orders as the Guadalupe swiftly rose above major flood stage.

"This happened very quickly, over a very short period of time that could not be predicted, even with radar," Rice said. "This happened within less than a two-hour span."

State emergency management officials had warned as early as Thursday that west and central Texas faced heavy rains and flash flood threats "over the next couple days," citing National Weather Service forecasts ahead of the holiday weekend.

But the weather forecasts in question "did not predict the amount of rain that we saw," W. Nim Kidd, director of the Texas Division of Emergency Management, told a news conference on Friday night.