Ohio shooting: Gunman kills 9 in Dayton's Oregon District

The attack comes hours after a gunman killed 20 in El Paso, Texas

Authorities work at the scene of a mass shooting on Sunday, Aug. 4, 2019, in Dayton, Ohio. AP Photo

A gunman killed at least nine people before being shot dead by police in the US city of Dayton in Ohio state, local police said on Sunday, in the second mass shooting in the country in 24 hours.

On Saturday, 20 people were killed in the Texas town of El Paso in a shooting in a Walmart store.

Less than 13 hours later, an unnamed gunman wearing body armour and using an "AK-like assault rifle" with "high-capacity magazines" opened fire in the popular bar and nightlife district called Oregon, Mayor Nan Whaley said.

She said at least 27 more people were treated for injuries, and the gunman was taken out in "less than one minute".

"Fortunately we had officers in the immediate vicinity when this incident started so there was a very short timeline of violence. For that we're very fortunate," Police Lt Col Matt Carper said.

"As bad as this is, it could have been much, much worse."

The shooter, identified as Connor Betts, 24, died after being shot by police.

The suspect had opened fire on the street firing "a long gun with multiple rounds".

Police confirmed on Sunday afternoon that Betts's sister was among those killed.

Lt Col Carper said Oregon was "a very safe part of downtown" Dayton and very popular with visitors.

Sunday's shooting was the 22nd mass killing of 2019 in the US, a database from AP, USA Today and the Northeastern University shows. That amounts to a mass killing every 10 days.

Mass murders are classified as attacks where four or more people are killed, not including the offender.

Meanwhile, police and FBI investigators in Texas searched for clues on Sunday to explain what drove a young gunman from Dallas to kill 20 people at a Walmart store hundreds of kilometres away in the border city of El Paso.

The gunman was identified as Patrick Crusius, 21, from Allen, which is a 10-hour drive from El Paso.

He had reportedly posted a long message shortly before the attack, railing against the "Hispanic invasion" of Texas.

More than eight in 10 residents of the city, which was once part of Mexico, are of Hispanic descent.

Texas Governor Greg Abbott said Saturday morning's rampage appeared to be a hate crime.

On Twitter, US President Donald Trump called the shooting "an act of cowardice".

"I know that I stand with everyone in this country to condemn today's hateful act," Mr Trump said.

"There are no reasons or excuses that will ever justify killing innocent people."

Days earlier, a teenager killed three people with an assault rifle and injured a dozen others before taking his own life in a hail of police gunfire at a food festival in California.

Pope Francis condemned the attacks on "defenceless people".

Speaking to thousands in St Peter's Square for his Sunday message and blessing, the Pope said he was spiritually close to the victims, the wounded and the families affected by attacks he said had "bloodied Texas, California and Ohio".