Thunderstorms and torrential rain triggered deadly flash flooding along the Guadalupe River in Texas on Friday, killing at least 13 people and leaving more than 20 girls from a summer camp missing, according to local authorities.

The region was beset by death and disaster on Friday after months’ worth of heavy rain fell in a matter of hours, leaving search teams to conduct boat and helicopter rescues in the fast-moving water that overtook riverfront communities and children’s summer camps.

The US National Weather Service declared a flash flood emergency for parts of Kerr county, located in south-central Texas Hill Country, following the heavy downpours.

The Kerr county sheriff’s office reported 13 people were found dead in “catastrophic flooding” in the area.

The Texas lieutenant governor Dan Patrick said that 23 children from Camp Mystic, a private Christian summer camp for girls, were unaccounted for of 750 who were staying there at the time.

He asked people to stay away from the area, saying the Texas division of emergency management had 14 helicopters and hundreds of emergency workers involved in search-and-rescue operations.

Patrick said emergency crews had recovered between six and 10 bodies. “Some are adults, some are children. At this point, we don’t know where they all came from,” he said.

He said Donald Trump had been informed of the situation and responded: “Whatever we need, we can have.”

The region was inundated when five to 10in of rain fell overnight as part of an intense, slow-moving storm across Kerr and Kendall counties. The runoff from parched land caused the Guadalupe River to crest at one of its highest-ever peaks, with water levels in Kerrville rising more than 22ft in just a few hours overnight.

Teams have conducted dozens of rescues as the emergency response continued and an unknown number of people remained unaccounted for. The state senator Pete Flores said: “We are in search-and-rescue mode, and we know that these first 24 hours are very important.”

Dalton Rice, city manager for Kerrville, the county seat, told reporters the extreme flooding struck with little to no advance warning, precluding authorities from issuing any evacuation orders.

“This happened very quickly, over a very short period of time, that could not be predicted, even with the radar,” Rice said. “This happened within less than a two-hour span.”

Of particular concern are more than a dozen summer camps dotted in the rural region that would now be filled with kids.