At least 80 people have been confirmed dead following flash floods in central Texas, authorities said on Sunday, as rescuers continued a frantic search for dozens of campers, vacationers and residents who were still missing.

At least 28 of the confirmed dead were children, many of whom were attending Camp Mystic, a Christian summer camp for girls. A few hundred children were attending the camp.

The flood hit early in morning on Friday, Independence Day, after a sudden rain dumped up to 15 inches (38 centimeters) of rain across the region. Some sections of the Guadalupe River rose by more than 6 meters, leading to the riverbank overflowing, and sweeping away trailer houses as people were sleeping.

Rains continued pounding communities outside San Antonio on Saturday and flash flood warnings and watches remained in effect.

Texas authorities advised people in the area to seek higher ground, and said that more flooding was possible.

Searchers used helicopters, boats and drones to look for victims and to rescue people stranded in trees and from camps isolated by washed-out roads.

Another camp by Heart O' the Hills announced its director and co-owner Jane Ragsdale also died in the flooding. Luckily, all the children attending that camp got out in time.

A local soccer coach, his wife and their two children perished in the flood. A 27-year-old man managed to save his family, including his mother, fiancee and their children, but died in the effort.

A state official said more than 800 people have been rescued so far. Texas Governor Greg Abbott has issued a disaster declaration for 15 counties in the region.

"I urge every Texan to join me in prayer this Sunday — for the lives lost, for those still missing, for the recovery of our communities and for the safety of those on the front lines," he said in a statement.

While vowing continuous search operations until everyone missing is accounted for, officials did acknowledge that as time goes by, the chances of finding survivors would diminish.

Kerr County Judge Rob Kelly said on Friday that "no one knew this kind of flood was coming". A flood warning sounded in the dead of night shortly after 1 am. People were caught off guard.

City Manager Dalton Rice of Kerrville said the extent and scale of the flood was unprecedented.

"From our flood maps, we're seeing some areas where it's hitting the 100-year flood (level)," Rice said on Saturday evening.

Kerr County and its surrounding region is part of Texas Hill Country, known for its natural beauty, but it also has a history of flash floods. Many came and camped along the Guadalupe River to celebrate Independence Day.

"I would just say this is a 100-year catastrophe and it's just so horrible to watch," US President Donald Trump said, adding that he may visit the area later in the week.

"God bless all of the people that have gone through so much," he said.

Agencies contributed to this story.