A gunman wearing body armour and carrying extra magazines opened fire in Dayton, Ohio, killing nine and injuring dozens, authorities have said.

It was the second mass shooting in the US in less than 24 hours.

Dayton police patrolling the area responded in less than a minute to the shooting, which unfolded at about 1am on Sunday on the streets of the Oregon District, Dayton Mayor Nan Whaley said at a press conference.

Ms Whaley said if the police had not responded so quickly, “hundreds of people in the Oregon District could be dead today”.

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.

The gunman, who has not been identified by authorities, was shot to death by responding officers.

Ms Whaley said he was carrying a .223-calibre rifle and had additional high-capacity magazines with him.

Police believe there was only one shooter and have not yet identified the suspect or a motive.

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.

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Dayton police look for evidence after the mass shooting (Marshall Gorby/Dayton Daily News/AP)

Miami Valley Hospital spokeswoman Terrea Little said the hospital had received 16 victims but she could not confirm their conditions.

Kettering Health Network spokeswoman Elizabeth Long said multiple victims from a shooting had been brought to system hospitals, but she didn’t have details on how many.

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.”

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Witnesses comfort one another at the scene in Dayton (John Minchillo/AP)

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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Police tape drapes over a pile of shoes (Marshall Gorby/Dayton Daily News/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”.

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.