This is the second such incident in US in 24 hours. (Representational)

Nine people were killed in a mass shooting early Sunday in Dayton, Ohio, police said, adding that the assailant was shot dead by responding officers.

The incident occurred shortly after 1:00 am in the popular bar and nightlife Oregon district of the city, Police Lieutenant Colonel Matt Carper said.

"We had one shooter that we are aware of and multiple victims," he told reporters.

"The shooter is deceased, from gunshot wounds from the responding officers," he said, adding no police were injured.

"We have nine victims deceased ... and we have approximately 16 more victims hospitalized right now in unknown conditions."

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

Police were working to identify the shooter and the FBI were on scene to provide any necessary assistance, he said.

The shooting came just hours after 20 people were killed by a gunman in El Paso, Texas.

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

"Fortunately we had multiple officers in the immediate vicinity when this incident started so there was a very short timeline of violence, for that we've very fortunate.

"It's a very tragic incident and we're doing everything we can to investigate it and try to identify the motivation behind this," said Carper.

"We're still interviewing obviously a lot of witnesses as well as officers to determine if anyone else was involved," he said.

This is the second such incident reported in the US in 24 hours. A gunman armed with an assault rifle opened fire on shoppers at a packed Walmart store in El Paso, Texas on Saturday, killing 20 people.

Authorities in Texas are investigating the mass shooting as a possible hate crime, the police chief said. A 21-year-old white male suspect from Allen, a suburb of Dallas, surrendered to police outside the store after the rampage.