At least nine people have been killed and at least 26 are injured after a shooting in Dayton, Ohio on Sunday morning (local time), US police have said.

Key points: The shooting reportedly took place in Ohio's Oregon District, an area renowned for its bars, restaurants and theatres

The shooting reportedly took place in Ohio's Oregon District, an area renowned for its bars, restaurants and theatres The shooting is the 22nd mass killing in the US this year, and came hours after a shooting in El Paso, Texas

The shooting is the 22nd mass killing in the US this year, and came hours after a shooting in El Paso, Texas The FBI was assisting officials with the investigation

The alleged shooter was shot dead by responding officers, Lieutenant Colonel Matt Carper said at a press conference.

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

Lieutenant Colonel Carper said the suspect used a long gun and fired multiple rounds.

Dayton police patrolling the area responded in less than a minute to the shooting, Dayton Mayor Nan Whaley said at a subsequent media briefing.

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

The Mayor said the alleged gunman was carrying a .223-calibre rifle and had high-capacity magazines with him.

Dayton Police tweeted that an active shooter situation began in the Oregon District at 1:00am (local time), but that officers nearby were able to "put an end to it quickly", Lieutenant Colonel Carper said.

The Lieutenant Colonel described the Oregon District as a "safe part of downtown".

Miami Valley Hospital spokeswoman Terrea Little said 16 victims have been received at the hospital, 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 did not have details on how many.

"This is extremely unusual, obviously, for any community, let alone Dayton. In our Oregon District, this is unheard of," Lieutenant Colonel Carper said.

Police have not said where in the Oregon District the shooting took place. ( AP: John Minchillo )

The FBI was assisting with the investigation.

Witnesses describe seeing bodies on the ground

Two women who were in a bar across the street said they were in shock after witnessing the shooting.

Nikita Papillon said she heard the shooting and ran out of Newcom's Tavern, where she saw bodies on the ground, including that of a woman she had talked to earlier.

"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.

The 23-year-old said people her age "don't think something like this is going to happen... and when it happens, words can't describe it".

Tianycia Leonard said she heard "loud thumps" she initially thought were people pounding on a dumpster. The 28-year-old said she then realised "it was gunshots and there were a lot of rounds".

Witnesses comfort one another at the scene of the mass shooting. ( AP: John Minchillo )

Police tweeted a family assistance centre was set up at the Dayton Convention Centre, shortly after issuing a call for information about the shooting.

"I'm heartbroken," Ms Whaley tweeted around 6:00am on Sunday, thanking first responders for their efforts.

Ohio Governor Mike DeWine issued his own statement before 7:00am, announcing he had ordered flags in Ohio remain at half-mast and offered assistance to Ms Whaley.

"Fran and I are absolutely heartbroken over the horrible attack that occurred this morning in Dayton," the statement said.

"We join those across Ohio and this country in offering our prayers to victims and their families."

The Oregon District is a historic neighbourhood near downtown Dayton that is home to bars, restaurants and theatres.

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Police have not said where in the district the shooting took place, but said it comprised a "large scene and investigation".

The shooting came hours after a shooter killed at least 20 people at a Texas Walmart packed with back-to-school shoppers.

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.

At least 16 victims have been taken to local hospitals. ( AP: John Minchillo )

The El Paso shooting was the 21st mass killing in the United States in 2019, according to the AP/USATODAY/Northeastern University mass murder database that tracks all US homicides since 2006 involving four or more people killed — not including the offender — over a short period of time regardless of weapon, location, victim-offender relationship or motive.

That makes the shooting in Dayton the 22nd mass killing in the US this year.

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

AP