A 64-year-old man holed up in a 32nd-floor hotel room with multiple machine guns 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 of bullets from the Mandalay Bay hotel into a crowd of 22,000 people lasted several minutes, causing panic. At least 515 people were injured as some fleeing fans trampled each other while police scrambled to locate the shooter.

Police on Monday identified the gunman as Stephen Paddock (photo), who lived in a retirement community in Mesquite, Nevada. They said they believed he acted alone and did not know why he attacked the crowd. The Islamic State militant group claimed responsibility for the massacre, but US officials said there was no evidence of that.

The preliminary death toll, which officials said could rise, eclipsed last year's massacre of 49 people at an Orlando, Florida, nightclub by a gunman who pledged allegiance to IS 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 Paddock had no criminal record. The gunman 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 militant organisations.

"We have determined to this point no connection with an international terrorist group," Federal Bureau of Investigation Special Agent in Charge Aaron Rouse told reporters.

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

"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."

Multiple machine guns

Lombardo said there were more than 10 rifles in the room where Paddock killed himself. His arsenal included multiple machine guns, according to a law enforcement official. US law largely bans machine guns.

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

The shooting, just the latest in a string that have played out across the US over recent years, sparked an outcry from some lawmakers about the pervasiveness of guns in the US, but was unlikely to prompt action in Congress.

Efforts to pass federal laws on guns failed following mass shootings from the 2012 massacre of 26 young children and educators in Newtown, Connecticut, to the June attack on Republican lawmakers practicing for a charity baseball game.

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

House of Representatives Minority Leader Nancy Pelosi, a Democrat, on Monday called on House Speaker Paul Ryan to create a select committee on gun violence.

"Congress has a moral duty to address this horrific and heartbreaking epidemic," Pelosi wrote.

The Second Amendment of the US Constitution protects the right to bear arms, and gun-rights advocates staunchly defend that provision. US President Donald Trump, a Republican, has been outspoken about his support of the Second Amendment.

"To all those political opportunists who are seizing on the tragedy in Las Vegas to call for more gun regs. You can't regulate evil," Kentucky's Republican Governor, Matt Bevin, said on Twitter.

The dead in Las Vegas included a nurse, a government employee and an off-duty police officer.

Trump said he would travel to Las Vegas on Wednesday to meet with first responders.

"It was an act of pure evil," Trump said in a White House address.

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

"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."

'Just kept going on'

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 some 3.5 million visitors from around the world each year and the area was packed with visitors when the shooting broke out shortly after 10pm local time.

Shares of MGM Resorts International, which owns the Mandalay Bay, fell almost five percent on Monday to US$30.98 a share.

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.

- Reuters