Las Vegas police sweep through a convention center area during a lockdown at the Tropicana Las Vegas following an active shooter situation on the Las Vegas Strip. Picture: Chase Stevens/Las Vegas Review-Journal via AP

Las Vegas - A 64-year-old man armed with more than 10 rifles rained down gunfire on a Las Vegas country music festival on Sunday, slaughtering at least 58 people in the largest mass shooting in US history before killing himself.

The barrage from a 32nd-floor window in the Mandalay Bay hotel into a crowd of 22 000 people lasted several minutes, causing panic. Some fleeing fans trampled each other as police scrambled to find the gunman. More than 500 people were injured.

On Monday, police identified the gunman as Stephen Paddock, who lived in a retirement community in Mesquite, Nevada, and said they had no sense of what prompted his attack. The Islamic State militant group claimed responsibility for the massacre, but US officials expressed skepticism of that claim.

The preliminary death toll, which officials said could rise, eclipsed last year's massacre of 49 people at an Orlando night club by a gunman who pledged allegiance to Islamic State militants.

Shocked concertgoers, some with blood on their clothing, wandered streets, where the flashing lights of the city's gaudy casinos blended with those of emergency vehicles.

Police said they had no information about Paddock's motive, that he had no criminal record and was not believed to be connected to any militant group. Paddock killed himself before police entered the hotel room he was firing from, Clark County Sheriff Joseph Lombardo told reporters.

"We have no idea what his belief system was," Lombardo said. "I can't get into the mind of a psychopath."

Federal officials said there was no evidence to link Paddock to international militant groups.

"We have determined to this point no connection with an international terrorist group," FBI special agent in charge Aaron Rouse told reporters.

US officials discounted a claim of responsibility for the attack made by the Islamic State, through its Amaq news agency.

Picture: Amaq News Agency via AP

"The Intelligence Community is aware of the claim of responsibility by a foreign terrorist organisation for the shooting in Las Vegas," CIA spokesman Jonathan Liu said in an email. "We advise caution on jumping to conclusions before the facts are in."

One US official said there was reason to believe that Paddock had a history of psychological problems.

Lombardo said there were more than 10 rifles in the room where Paddock killed himself. He had checked into the hotel on Thursday.

Police found several more weapons at Paddock's home in Mesquite, about 145 km north-east of Las Vegas, Mesquite police spokesman Quinn Averett told reporters.

Nevada has some of the most permissive gun laws in the United States. It does not require firearm owners to obtain licenses or register their guns.

The dead included one off-duty police officer, Lombardo said.

"He brutally murdered more than 50 people and wounded hundreds more. It was an act of pure evil," US President Donald Trump said in a White House address. He ordered flags lowered to half-staff in a national gesture of mourning and said he would visit Las Vegas on Wednesday.

Leaders from around the world expressed shock and sadness at the news.

Video of the attack showed panicked crowds fleeing as sustained rapid gunfire ripped through the area.

"People were just dropping to the ground. It just kept going on," said Steve Smith, a 45-year-old visitor from Phoenix, Arizona. He said the gunfire went on for an extended period of time.

"Probably 100 shots at a time," Smith said. "It would sound like it was reloading and then it would go again."

Las Vegas's casinos, nightclubs and shopping draw about 3.5 million visitors from around the world each year and the area was packed with visitors when the shooting broke out.

Shares of US casino operators fell in morning trading on Wall Street, with MGM Resorts International, which owns the Mandalay Bay, down 3.5 percent. Melco Resorts & Entertainment Ltd, Wynn Resorts Ltd and Las Vegas Sands Corp were little changed.

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Mike McGarry, a financial adviser from Philadelphia, was at the concert when he heard hundreds of shots ring out.

"It was crazy - I laid on top of the kids. They're 20. I'm 53. I lived a good life," McGarry said. The back of his shirt bore footmarks, after people ran over him in the panicked crowd.

The shooting broke out on the final night of the three-day Route 91 Harvest festival, a sold-out event featuring top acts such as Eric Church, Sam Hunt and Jason Aldean.

The suspected shooter's brother, Eric Paddock, said the family was stunned.

"We have no idea. We're horrified. We're bewildered and our condolences go out to the victims," Eric Paddock said in a phone interview, his voice trembling. "We have no idea in the world."

As with previous US mass shootings, the incident sparked anger among advocates for gun control. The Second Amendment of the US Constitution protects the right to bear arms, and gun-rights advocates staunchly defend that provision.

"It's time for Congress to get off its ass and do something," said US Senator Chris Murphy of Connecticut, where 26 young children and educators were killed in an attack on a school in 2012.

Efforts to pass federal legislation after that attack failed. Gun rights advocates argued that restrictions on legal gun sales would leave law-abiding citizens more vulnerable to attacks by criminals.