Some 27 people, including nine children, have been confirmed dead after flash floods in the central of the US state of Texas, authorities said on Saturday, as rescuers continued a frantic search for survivors, including dozens still missing from a girls’ summer camp.

The sheriff’s office in Kerr County, Texas, said more than 800 people had been evacuated from the region as flood waters receded in the area around the Guadalupe River, about 137 kilometres northwest of San Antonio.

“We will not stop until every single person is found,” Kerr County Sheriff Larry Leitha said at a news conference.

At least 23 to 25 people from the Camp Mystic summer camp were missing, most of them reported to be young girls. The river waters rose 29 feet rapidly near the camp.

The US National Weather Service said that the flash flood emergency has largely ended for Kerr County, the epicentre of the flooding, following thunderstorms that dumped as much as a foot of rain early on Friday. A flood watch, however, remained in effect until 7pm for the broader region.

Kerr County sits in the Texas Hill Country, a rural area known for its rugged terrain, historic towns and other tourist attractions.

Texas Lieutenant Governor Dan Patrick said up to 500 rescue workers were searching for an unknown number of people who were still missing, including many who had come to the area for an Independence Day celebration by the river.

“We don’t know how many people were in tents on the side, in small trailers by the side, in rented homes by the side, because it was going to be the Fourth of July holiday,” he said on Fox News Live.

US President Donald Trump said the federal government was working with state and local officials to respond to the flooding. “Melania and I are praying for all of the families impacted by this horrible tragedy. Our brave first responders are on site doing what they do best,” he said on social media.

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

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

The forecasts, however, “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.

The weekend disaster echoes a catastrophic 1987 Guadalupe River flood in which 10 teenagers drowned when trying to leave a church camp, according to the National Weather Service.