Devastating flash floods have killed at least 25 people in the Appalachia region of eastern Kentucky - the worst such disaster there for decades.
Kentucky Governor Andy Beshear said he expected the death toll to continue to rise. He said hundreds of homes and businesses had been flooded.
President Joe Biden has declared the floods "a major disaster" and ordered federal aid to help local rescuers.
Among the dead are at least six children, including a one-year-old.
Scientists say climate change is triggering more extreme weather events like the Kentucky flooding.
After an aerial tour with the National Guard, Mr Beshear said the flood was "by far the worst" he had seen.
"There's still a lot of people out there, still a lot of people unaccounted for. We're going to do our best to find them all," he added, warning that it could take weeks to find all the victims.
Hundreds of people have been rescued by boat or helicopter.
There is widespread poverty in affected areas, where at least 33,000 people now have no electricity. The flooding - after torrential rain - caused mudslides and made roads impassable.
Appalachia has had flash floods before, but not on this scale, Mr Beshear said.
"Folks who deal with this for a living, who have been doing it for 20 years, have never seen water this high," he said.
Some areas reported more than eight inches (20cm) of rain in a 24-hour period.
Perry and Knott counties are especially hard hit, and there has been some flooding too in neighbouring Virginia and West Virginia. Rivers in the region are expected to crest throughout the weekend.
More rainfall is expected early next week, Mr Beshear said, urging people to have a safety plan in place.
Representatives from the US Federal Emergency Management Agency arrived on Friday to assist local rescue efforts.
Mr Beshear said it's unclear yet exactly how many people are affected or missing. Some areas have been difficult to reach due to damaged portions on 28 state roadways.
He confirmed that the bodies of four young siblings were among those who were found - the oldest an eight-year-old and the youngest a one-year-old.
The children's cousin - Brittany Trejo - told the Lexington Herald-Leader that the children and their parents "managed to get to a tree", but later "a big tide came and wash them all away at the same time".
She added the mother and father, who survived, were stranded at the tree for eight hours before help arrived.
Mr Beshear said "it's going to be a tough couple of days, it's going to be a long rebuild".
Kentucky, like other parts of the world, has seen the impact of more frequent extreme weather events. The state has seen more inches of rain outside the historical average in the last 10 years, according to date from the National Oceanic and Atmospheric Administration.
Bill Haneberg, a climate expert and the state's geologist, said this rainfall event is "extraordinary" for Kentucky.
He added that the increases in the amount of rainfall over the years are consistent with what experts have predicted for the region - that Kentucky's climate would become hotter and wetter due to climate change.
The historic flooding comes as the state recovers from the deadliest tornadoes in its history, which killed more than 70 in December 2021.