The death toll from the floods in central Texas has risen to 43 from 32, Kerr County Sheriff Larry Leitha has said at a news conference.
Among the missing are 27 young girls from the Camp Mystic summer camp.
Kerrville City Manager Dalton Rice said there could be more people missing in the region.
"We are kind of looking at this in two ways called the known missing, which is the 27... We will not put a number on the other side because we just don't know," he said at the same press conference.
Addressing a press conference, Texas Governor Greg Abbott said he was expanding a state disaster declaration and was requesting additional federal resources from US President Donald Trump.
Texas Department of Emergency Management chief Nim Kidd said air, ground and water-based crews were scouring the length of the Guadalupe River for survivors and the bodies of the dead.
"We will continue the search until all those who are missing are found," he said.
The flooding began on Friday as months' worth of rain fell in a matter of hours, causing the Guadalupe River to rise by 26 feet (eight metres) in 45 minutes.
The National Weather Service warned that more rain was forecast, and that "excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations."
In Kerrville on Saturday, the usually calm Guadalupe was flowing fast, its murky waters filled with debris.
"The water reached the top of the trees. About 10 metres or so," said local resident Gerardo Martinez, 61. "Cars, whole houses were going down the river."
Flash floods, which occur when the ground is unable to absorb torrential rainfall, are not unusual.
But scientists say that in recent years human-driven climate crisis has made extreme weather events like floods, droughts and heat waves more frequent and more intense.
Devastation at Camp Mystic
On Saturday, Sheriff Leitha said 27 children from the Camp Mystic Christian summer camp in the flooded Kerr County were still missing. Around 750 girls were enrolled at the camp.
US media reported that four of the missing girls were dead, citing their families.
The camp, located along the banks of the Guadalupe, was a picture of disarray, with blankets, mattresses, teddy bears and other belongings scattered across buildings.
The windows of camp cabins were shattered, apparently by the force of the water.
The Heart O' The Hills summer camp, located about a mile away from Camp Mystic, confirmed on Saturday that its director Jane Ragsdale was among the dead.
‘A hard day’
Kerrville city official Dalton Rice said rescuers were facing "very difficult" conditions.
"We did start boots on the ground operations about 8:00 am (1300 GMT) this morning," he said, warning residents not to launch their own searches.
Rice added that it was not known how many people may have been visiting the popular camping area, and declined to give an overall figure for how many people were missing.
State and local officials warned against residents travelling to the area, which includes campgrounds dotted along the river, with dozens of roads impassable.
Videos on social media showed houses and trees swept away by the flash flood caused by heavy overnight rain of up to 12 inches, one-third of Kerr County's average annual rainfall.
With rescuers fanning out across the region, Joe Herring, the Kerrville mayor, urged the community to come together.
"People need to know today will be a hard day. It will be a hard day," he said, his voice breaking.
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Source: TRT