At least 20 killed in Las Vegas shooting

The gunman, who police said was a local resident, was killed after being "engaged" by officers who responded to report of multiple gunfire coming from the 32nd floor of the Mandalay Bay, a hotel-casino next to the concert venue.

Revelers screamed and fled in panic as a steady stream of automatic gunfire rang out at the venue shortly after 10:00pm local time, footage captured on smart phones showed.

"We have in excess of 100-plus injured and excess of 20-plus that have died at this point," Las Vegas Metro Police Sheriff Joseph Lombardo told an early morning press conference.

"This is an ongoing investigation, but we are comfortable that the primary aggressor in this event is - has expired or passed away, and is no longer a threat."

Thousands of fans were attending the concert next to the Mandalay Bay which was part of a three-day country music festival known as Route 91.

ALSO READ: Police nab one Las Vegas shooting suspect

Witnesses told how the gunman opened fire with an inital burst and then appeared to reload as he continued his spree. 

"We heard (what) sounded like a glass breaking, so you looked around to see what's going on and then heard a pop, pop, pop," Monique Dekerf told CNN.

"You'd think for a moment okay we're fine, there's no more gunfire, then it starts again."

Her sister Rachel said it sounded like "the shots were coming from the right side ... it sounded like they were right beside us too ... it was right there."

Best-selling country singer, Jason Aldean was on stage at the time of the shooting which came towards the end of the concert.

"We were watching the concert having a great time, then we heard what sounded like firecrackers," witness Joe Pitz told the local Las Vegas Sun news outlet.

"I guess it was an automatic weapon going off but it literally sounded like firecrackers.

"Then soon enough there was commotion on the Mandalay Bay side of the stage. They were motioning for medics to come and safety people to come and Jason Aldean ran off the stage.

"Everybody in the vicinity went down. I don't know if they were ducking or if they were but it was chaos."

'Like firecrackers' 

Lombardo would not release the name of the gunman, but said he was a local man and issued an appeal for information about his companion.

"He has been identified, he is a local resident," he said at the press conference.

Lombardo said he will not release at this time, but police are interested in his traveling companion.

"We have not located her at this time and we are interested in talking to her for follow-up," Lombardo said.

"We are pretty confident there is no longer a threat other than we are attempting to locate the person of interest.

"It's on ongoing investigation. Right now, we believe he is the sole aggressor at this point."

A shocked Aldean told his fans via Instagram that he and his band was safe.

"Tonight has been beyond horrific," the singer wrote. 

"I still dont know what to say ... My Thoughts and prayers go out to everyone involved tonight.

"It hurts y heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."