Thirteen people have died in Kerr County in the US state of Texas, according to authorities, after severe weather and flooding hit parts of the state.

More people are unaccounted for, including at a summer camp where Texas Lieutenant Governor Dan Patrick confirmed around 20 children are missing.

"Within 45 minutes, the Guadalupe River rose 26 feet and it was a destructive flood, taking property and sadly lives," he said at a news conference on Friday.

He read out a statement from all-girls Camp Mystic, where about 750

• Follow live updates

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," the lieutenant governor, who is acting governor while Governor Greg Abbott is on vacation, 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.

Another official confirmed that the search will continue through the night.

He said the flooding affected an area that has a lot of summer camps that "thousands" of children attend, especially over the 4 July Independence Day weekend.

"We have not had reports from other camps at this point of a loss, but that does not mean there has not been," Patrick said.

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 the 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.

Governor Greg Abbott said Texas was providing "all necessary resources to Kerrville, Ingram, Hunt and the entire Texas Hill Country dealing with these devastating 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.