A gunman in body armour who opened fire in a popular entertainment district in Dayton, Ohio, killed nine people including his sister, officials have said.

Connor Betts, 24, was shot dead by police less than a minute after he opened fire with a .223-calibre rifle in the streets of the Oregon District at around 1am on Sunday in the second US mass shooting in less than 24 hours.

Police have not released further information about Betts or publicly discussed a motive.

The official who identified the gunman spoke to The Associated Press on condition of anonymity.

His 22-year-old sister Megan was the youngest of those killed.

The other men and women who were killed ranged in age from 25 to 57.

Dayton Mayor Nan Whaley said the gunman was wearing body armour and had additional high-capacity magazines.

Had police not responded so quickly, “hundreds of people in the Oregon District could be dead today”, she said.

The Oregon District is a historic neighbourhood that Lieutenant Colonel Matt Carper described as “a safe part of downtown,” home to bars, restaurants and theatres.

Ms Whaley said at least 27 people were treated for injuries and at least 15 of those have been released.

Several more remain in serious or critical condition, local hospital officials said at a news conference.

They said some people suffered multiple gunshot wounds and others suffered injuries as they fled.

Ohio Shooting
Officers walk among evidence markers at the scene of the mass shooting (John Minchillo/AP)

Nikita Papillon, 23, was across the street at Newcom’s Tavern when the shooting started.

She said she saw a girl she had talked to earlier lying outside Ned Peppers Bar.

“She had told me she liked my outfit and thought I was cute and I told her I liked her outfit and I thought she was cute,” Ms Papillon said.

She had been to Ned Peppers the night before, describing it as the kind of place “where you don’t have to worry about someone shooting up the place.”

“People my age, we don’t think something like this is going to happen,” she said.

“And when it happens, words can’t describe it.”

Tianycia Leonard, 28, was in the back of Newcom’s.

She heard “loud thumps” she initially thought was people pounding on a skip.

“It was so noisy but then you could tell it was gunshots and there was a lot of rounds,” Ms Leonard said.

Ned Peppers staff said in a Facebook post they were left shaken and confused by the shooting.

The bar said a bouncer was treated for shrapnel wounds.

A message seeking further comment was left with staff.

President Donald Trump has been briefed on the shooting and praised law enforcement’s speedy response in a tweet on Sunday morning.

Governor Mike DeWine issued his own statement before 7am, announcing he has ordered flags in Ohio remain at half-mast,.

He also offered assistance to Ms Whaley.

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Residents comfort each other as they await word on whether they know any of the victims (Julie Carr Smyth/AP)

The FBI is assisting with the investigation.

A family assistance centre was set up at the Dayton Convention Centre, where people seeking information on victims arrived in a steady trickle throughout the morning.

Some local pastors were on hand to offer support, as were comfort dogs.

Ms Whaley said the Oregon District is expected to reopen on Sunday afternoon and a vigil is planned on Sunday evening.

The minor league baseball team Dayton Dragons, who play in nearby Fifth Third Field, postponed their Sunday afternoon game against the Lake County Captains “due to this morning’s tragic event”.

US Mall
(PA Graphics)

The Ohio shooting came hours after a young man opened fire in a crowded shopping area in El Paso, Texas, leaving 20 dead and more than two dozen injured.

Just days before, on July 28, a 19-year-old shot and killed three people, including two children, at the Gilroy Garlic Festival in northern California.

Sunday’s shooting in Dayton was the 22nd mass killing in the US this year.

The first 20 mass killings in the US in 2019 claimed 96 lives.

The shooting in Dayton comes after the area was heavily damaged when tornadoes swept through western Ohio in late May, destroying or damaging hundreds of homes and businesses.

“Dayton has been through a lot already this year and I continue to be amazed by the grit and resiliency of our community,” Ms Whaley said.