A gunman opened fire in Ohio's popular bar and nightlife district called. (AFP)

A gunman who shot dead nine people in the city of Dayton, Ohio, in the latest US mass shooting killed his victims in less than a minute, the mayor said Sunday.

Mayor Nan Whaley told a press conference that the unnamed gunman was wearing body armour and used an "AK-like gun assault rifle" with "high capacity magazines" when he opened fire in the popular bar and nightlife district called Oregon.

"In less than one minute, Dayton first responders neutralized the shooter," Whaley said.

As well as the nine fatalities, hospitals treated 27 people who were wounded in the shooting, the mayor said.