The death toll from catastrophic floods in Texas reached at least 78 on Sunday, including at least 28 children, as the search for girls missing from a summer camp entered a third day and fears of more flash flooding prompted fresh evacuations.

Larry Leitha, the Kerr County Sheriff in Texas Hill Country, said 68 people had died in flooding in his county, the epicentre of the flooding, among them 28 children.

Texas Governor Greg Abbott, speaking at a press conference on Sunday afternoon, said another 10 had died elsewhere in Texas and confirmed 41 were missing.

US President Donald Trump sent his condolences to the victims and said he would probably visit the area on Friday. His administration had been in touch with Abbott, he added.

Among the most devastating impacts of the flooding occurred at Camp Mystic summer camp, a nearly century-old Christian girls camp where 10 campers and one counsellor were still missing, according to Leitha.

"It was nothing short of horrific to see what those young children went through," said Abbott, who noted he toured the area on Saturday and pledged to continue efforts to locate the missing.

The flooding occurred after the nearby Guadalupe River broke its banks after torrential rain fell in the central Texas area on Friday, the US Independence Day holiday.

Officials said on Saturday that more than 850 people had been rescued, including some clinging to trees, after a sudden storm dumped up to 38cm of rain across the region, about 140km northwest of San Antonio.

The Federal Emergency Management Agency was activated on Sunday and is deploying resources to first responders in Texas after Trump issued a major disaster declaration, the Department of Homeland Security said.

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 themselves.

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.

Trump's administration has overseen thousands of job cuts from the National Weather Service's parent agency, the National Oceanic and Atmospheric Administration, leaving many weather offices understaffed, former NOAA director Rick Spinrad said.

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

Trump pushed back when asked on Sunday if federal government cuts hobbled the disaster response or left key job vacancies at the National Weather Service under Trump's oversight.

"That water situation, that all is, and that was really the Biden setup," he said referencing his Democratic predecessor Joe Biden. "But I wouldn't blame Biden for it, either. I would just say this is 100-year catastrophe."

He declined to answer a question about FEMA, saying only "They're busy working, so we'll leave it at that," Trump said. (Reuters)