At least 58 people have been killed and over 500 others injured when a gunman opened fire at a music concert in Las Vegas, Nevada, making it the deadliest mass shooting in modern US history.

The "nonstop gunfire" sent bystanders outside the resort scrambling for their lives while tourists hid in their hotel rooms. Disturbing footage showed the aftermath as the injured lay on stretchers or on the ground with responders and bystanders trying to provide assistance. 

The gunman rained down bullets from the 32nd floor of the Mandalay Bay Resort and Casino for several minutes before shooting himself dead, according to police. Earlier reports indicated that the gunman, identified as 64-year-old Stephen Paddock from Mesquite, Nevada, had been shot by police.

"Officers responded to that location and engaged the suspect at that location," Las Vegas Metro Police Sheriff Joseph Lombardo said.

Paddock had more than 10 rifles in his hotel room.

Lombardo said Paddock was not believed to be connected to any militant group.

"We have no idea what his belief system was," he said. "We’ve located numerous firearms within the room that he occupied."

However, the Daesh (ISIL) terrorist group later claimed responsibility for the shooting, according to the group’s news agency. It provided no evidence to back up its claim. The terror organization often claims such incidents.

At least 406 people were taken to area hospitals with injuries, the Las Vegas Metropolitan Police said. 

The death toll, which police emphasized was preliminary, eclipses last year’s massacre of 49 people at an Orlando night club. The dead included one off-duty police officer. Two officers were also injured. 

The Las Vegas Police Department advised people in a statement to stay away from Mandalay Bay Casino on the south end of the Vegas strip on Las Vegas Boulevard and across from McCarran International Airport.

"Confirming that one suspect is down. This is an active investigation. Again, please do not head down to the Strip at this time," LVMPD said in a tweet.

Thousands of fans were attending the concert next to the Mandalay Bay which was part of a three-day music festival. Witnesses said they thought fireworks were going off as part of the concert.

Special SWAT units were deployed to the scene of the shooting and police shut down the strip.

Gunman and his companion identified

The suspected gunman had reportedly checked in as a hotel guest. 

Police released the name of a female companion of the gunman, identified as Mari Lou Danley, 62.

According to authorities, Danley who was being sought as a person of interest has been located.

Witnesses said the gunman opened fire with an initial burst and then appeared to reload as he continued his firing spree.

Police also asked onlookers to avoid sharing "tactical positions" of its officers on the internet out of fear that it would endanger the operation.

Singer Jason Aldean was on stage when the shooting started. And it came towards the end of the concert.

"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," he said. 

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

White House Press Secretary Sarah Sanders said in a statement Monday morning that President Donald Trump had been briefed on the mass shooting.

"We are monitoring the situation closely and offer our full support to state and local officials," Sanders said. "All of those affected are in our thoughts and prayers."

Shortly after Sanders’ statement, Trump tweeted, "My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!"

Nevada politicians also responded to the massacre, denouncing the violence and offering thanks to first responders.

"Senseless, horrifying act of violence in Las Vegas tonight," Republican Senator Dean Heller said in a tweet. "Thankful for police and first-responders on the scene," Heller said.

Nevada Governor Brian Sandoval, a Republican, called the shooting a “tragic” and “heinous act of violence" and offered his prayers for the victims.