The shooting took place at the Borderline Bar and Grill in Thousand Oaks, California at around 11.30pm Tuesday, police said, presstv.ir reported.
"Ongoing active shooter incident reported at Borderline in Thousand Oaks,” Ventura County Fire Department said in a statement.
"Please stay away from the area. Active law enforcement incident. Multiple injuries reported. Details still being determined. Multiple ambulances requested."
Eyewitnesses told local media that the gunman, described as bearded and wearing a hat and glasses, shot a security guard at the front door before entering the building and throwing "smoke grenades all over the place".
The gunman was believed to be still at large and at least one police officer was reported to be injured in the incident.
A female cashier was said to be among the injured, according to reports.
Witnesses said the man was armed with a handgun and had already fired a dozen shots before people inside the bar started to escape by using chairs to smash the windows.
The video showed casualties being carried away from the bar by members of the public and the emergency services.
Thousand Oaks is located about 40 miles (64 km) from downtown Los Angeles, California’s largest city.
The United States loses around 33,000 people to gun violence every year.
The issue of gun violence has become all the more polarizing under President Donald Trump, a Republican whose presidential campaign was funded partially by the National Rifle Association (NRA).