At least 24 people have died and up to 25 people from a girls' summer camp remain missing after flash floods hit the US state of Texas on Friday.

US President Donald Trump called the tragedy, which happened on Independence Day, "shocking" and "terrible". Federal officials have pledged assistance, said Texas Governor Greg Abbott.

An overnight search and rescue operation was under way, Abbott said, though Texas officials could not confirm the number of people missing.

Water levels rose quickly before the river was flooded. "Within 45 minutes, the Guadalupe River rose 26 feet and it was a destructive flood, taking property and sadly lives," Texas Lieutenant Governor Dan Patrick said.

Patrick also informed parents that if they have not been contacted, their child is accounted for.

"That does not mean [the missing children] have been lost. They could be out of communication," he added.

Patrick acknowledged the offers of personal helicopters and drones from the public, saying they do not need any additional equipment to help with rescue efforts.

He said rescue agencies had 14 choppers, 12 drones, nine rescue teams, and swimmers in the water - a total of 400-500 people on the ground.

Later on Friday, Texas Maj Gen Thomas Suelzer said five helicopters - paired with rescue swimmers - were deployed, as well as "high-profile tactical military vehicles" to move evacuees out of high water areas to shelter. Some 237 people had been rescued, he said.

Wardens from the Texas Parks and Wildlife Department have reached Camp Mystic, where authorities say more than 20 girls have been unaccounted for since floodwaters swept through the area.

Telecommunication outages in the area have made it hard to contact many people in the region - including those at the summer camp.

On Friday morning, flash flooding in the state prompted disaster declarations for the Hill Country and Concho Valley regions.

In Kerr County, the sheriff's office has reported severe flooding with several people missing and confirmed loss of life.

At a news conference on Friday afternoon, Kerr County Judge Rob Kelly - the top official in the county - was pressed as to why the camps along the Guadalupe River weren't evacuated in advance.

"We didn't know this flood was coming. Rest assured, no one knew this kind of flood was coming," Kelly responded.

He later added "we do not have a warning system" in the area, which floods regularly.

He said what happened on Friday "far" surpassed a flood in 1987, which killed 10 teenagers on a church camp bus near the town of Comfort, south of Kerr County.

Rescues and evacuations have been under way since the early morning and there are warnings of more potential flash flooding to come in the state.

The state received several months worth of precipitation in a few hours, according to officials, leading to dangerous flash floods.

The region is to the north-west of the city of San Antonio.

Pictures show the deep flood waters swamping bridges and fast moving water swirling down roads.

"Folks, please don't take chances. Stay alert, follow local emergency warnings, and do not drive through flooded roads," Texas Department of Agriculture Commissioner Sid Miller said.

The Kerr County Sheriff's Office told residents near creeks, streams and the Guadalupe River to move to higher ground.

Kerrville Mayor Joe Herring said there was no indication the floods were going to be so devastating as the country does not have an early warning system in place, as reported by The Kerrville Daily Times.

The Kerrville Breaking News group on Facebook is a forum where locals often post restaurant recommendations, upcoming events and resources in the area.

On Friday, it was inundated with posts from families who have relatives unaccounted for from the flooding.

A pleading mother shared that she had not been able to get in touch with her daughter and son-in-law, whose home was swept away from a road near Kerrville Lake.

One woman in Austin, Texas posted that her grandparents living along the Guadalupe River had not been heard from since yesterday.

Separately, in New Jersey, authorities say at least three people died in the state following heavy rain and thunderstorms on Thursday night.

Among the three killed were two men, aged 79 and 25, who died after a tree fell onto their car in Plainfield during Thursday's severe storm.

A 44-year-old woman was also killed when a tree fell on her vehicle in North Plainfield. (BBC)

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