The LA Times reports the FBI’s Joint Terrorism Task Force had been dispatched to the scene.

Witnesses told The LA Times someone ran into the bar about 11.30pm, US time, and started firing what appeared to be a black pistol.

"He shot a lot, at least 30 times. I could still hear gunshots after everyone left,” said the person, who did not give a name.

Aubrey Ryan, 27, of Newbury Park, was in the front of the restaurant with at least 15 friends when a man came in and started shooting at the ceiling.

The gunman shouted something, but Ryan could not hear what he said. A friend threw her onto an outdoor balcony as the gunfire rang out so she could escape, said Ryan, who feared some of her friends were still inside the bar.

A distraught witness who was inside the bar with his stepson during the shooting told KACB-TV those inside the venue were mostly teens.

"There was just young poeple, 18, 19, 20, just having a great time, and this maniac came in and started shooting people for no reason at all. These poeple have never hurt anyone in their lives, they are just kids," he said.

"It was constant, until he changed clips, then there was a pause and then he reloaded... and he kept on going and then he reloaded and kept popping them off."

Ventura County Fire Department said on its website that firefighters and first responders were at the scene of a reported shooting in Ventura County in California. They say there are "multiple injuries".

The department advised nearby residents about midnight there was an "ongoing active shooter incident" at Borderline.

"Please stay away from area. Active law enforcement incident. Multiple injuries reported. Details still being determined. Multiple ambulances requested," the department wrote on Twitter.

Borderline is a country music bar that hosts live music events. A "College Country Nights" event was being held on Wednesday evening, according to the bar’s website.

More to come.