The death toll from catastrophic floods in Texas has reached at least 67, including 21 children, as the search for girls missing from a summer camp enters a third day.

Larry Leitha, the Kerr County Sheriff in Texas Hill Country, the epicentre of the flooding, said the country death toll had reached 59, including the 21 children.

Leitha said 11 girls and a counsellor remained missing from a summer camp near the Guadalupe River, which broke its banks after torrential rain fell in the central Texas area on Friday (local time), the US Independence Day holiday.

A Travis County official said four people had died from the flooding there, with 13 unaccounted for. Officials reported another death in Kendall County.

The Burnet County Sheriff’s office reported two fatalities.

A woman was found dead in her submerged car in the city of San Angelo in Tom Green County, the police chief said.

Leitha said 18 adults and four children were still pending identification in Kerr County. He did not say if they were included in the death count of 59.

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

It was unclear exactly how many people in the area were still missing.

“Everyone in the community is hurting,” Leitha said.

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

United States Coast Guard helicopters and planes were helping the search and rescue efforts, DHS said.

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 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 until 1pm local time.

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

The 11 missing girls and the counsellor were from the Camp Mystic summer camp, a nearly century-old Christian girls camp. It 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, one had a missing wall.

-AAP