Torrential rain has unleashed flash floods along the Guadalupe River in Texas, killing at least 24 people as rescue teams scramble to save dozens of victims trapped by high water or reported missing in the disaster.
Among the missing are 23 to 25 people listed as unaccounted for at an all-girls Christian summer camp on the banks of the rain-engorged Guadalupe, authorities said.
At a news conference late on Friday, almost 18 hours after the July 4 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 US National Weather Service declared a flash flood emergency for parts of Kerr County in south-central Texas Hill Country, about 100km 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 before 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.
July 4 fireworks displays ended up being cancelled in flood-stricken communities throughout the region, including Kerrville, where the waterfront site for Friday night's planned US Independence Day celebration was submerged by the rain-swollen river.
At Friday night's briefing, Kerr County Sheriff Larry Leitha said 24 flood-related fatalities had been confirmed, up from 13 tallied earlier in the day.
Lieutenant Governor Dan Patrick said on Friday afternoon that authorities were searching for 23 girls listed as missing from among more than 750 children at summer camp sites along the banks of the Guadalupe River when the area was inundated about 4am.
The missing campers had been attending Camp Mystic, a private Christian summer camp for girls.
"We're praying for all those missing to be found alive," Patrick said.
It was not clear whether anyone unaccounted for might have ended up among the dead victims tallied countywide by the sheriff.
Otherwise, all other campers were safe, authorities said, with campers being moved throughout the day.
Pressed by reporters why more precautions were not taken with stormy weather in the forecast, County Judge Rob Kelly, the top local elected official, insisted a disaster of such magnitude was unforeseen.
"We have floods all the time. This is the most dangerous river valley in the United States," Kelly said.Â
"We had no reason to believe this was going to be anything like what's happened here. None whatsoever."
In an alert on Thursday, the Texas Division of Emergency Management said it had increased its readiness level and "activated additional state emergency response resources" as parts of west and central Texas braced "for continued heavy rainfall and flash flooding threats heading into the holiday weekend".