The search for more than two dozen children missing from a girls' summer camp hit by flash floods in Texas entered a third day on Sunday, as rescuers faced the threat of more flooding and the death toll in the region reached at least 43.

Local officials warned the number of dead will likely rise and were due to give an update Sunday morning, as search and rescue teams raced to find 27 girls missing from a camp near the Guadalupe River, which broke its banks after torrential rain fell in central Texas on Friday, the U.S. Independence Day holiday.

Officials said more than 850 people had been rescued, including some clinging to trees, after a sudden storm dumped up to 38 centimetres of rain across Texas Hill Country, about 140 kilometres northwest of San Antonio. It was unclear how many people in the area were still missing.

Some experts questioned whether cuts to the federal workforce by the Trump administration, including to the agency that oversees the National Weather Service, led to a failure by officials to accurately predict the severity of the floods and issue appropriate warnings ahead of the storm.

Many U.S. weather offices understaffed

U.S. President Donald Trump and his administration have overseen thousands of job cuts at the National Weather Service's parent agency, the National Oceanic and Atmospheric Administration, leaving many weather offices understaffed, said former NOAA director Rick Spinrad.

He said he did not know if those staff cuts factored into the lack of advance warning for the extreme Texas flooding, but said they would inevitably degrade the agency's ability to deliver accurate and timely forecasts.

U.S. Homeland Security Secretary Kristi Noem, who oversees NOAA, said a "moderate" flood watch issued on Thursday by the National Weather Service had not accurately predicted the extreme rainfall and said the Trump administration was working to upgrade the system.

More rain was expected in the area on Sunday. The National Weather Service issued a flood watch for Kerr County, the epicentre of the disaster, until 1 p.m. local time.

The disaster unfolded rapidly on Friday morning as heavier-than-forecast rain drove river waters rapidly to as high as nine metres.

Texas Gov. Greg Abbott, a Republican, told a press conference on Saturday he had asked Trump to sign a disaster declaration, which would unlock federal aid for those affected. Noem said Trump would honour that request.

Trump has previously outlined plans to scale back the federal government's role in responding to natural disasters, leaving states to shoulder more of the burden.

WATCH | Search operations continue after Texas floods: 

Search continues for Texas flood victims as death toll rises

Texas Gov. Greg Abbott expanded the state's disaster declaration to include more counties on Saturday in the wake of catastrophic flash floods that have killed dozens. Rescuers used helicopters, boats and drones to look for the missing, including more than two dozen children from a girls camp.

At least 15 of the confirmed dead are children, local officials said. The 27 missing girls were from the Camp Mystic summer camp, a nearly century-old Christian girls' camp, which had 700 girls in residence at the time of the flood.

A day after the disaster struck, the camp was a scene of devastation. Inside one cabin, mud lines indicating how high the water had risen were at least 1.83 metres from the floor. Bed frames, mattresses and personal belongings caked with mud were scattered inside. Some buildings had broken windows, and one had a missing wall.