A lone gunman, identified by Las Vegas police as 64-year-old retiree Stephen Paddock, began firing from a hotel room on the 32nd floor of the neighbouring Mandalay Bay Resort and Casino into the 30,000 people below at the Route 91 Harvest festival.

More than 16 rifles were found in the hotel room with hundreds of rounds of ammunition, the New York Times reported. A further 18 weapons and explosives were discovered inside his home.

A state of emergency was declared in Clark County, Nevada following the shooting.

An Australian woman Marilou Danley, identified as a female companion who lived with Paddock, was overseas at the time of the shooting but will be interviewed by police when she returns, the Las Vegas Metropolitan Police Department said.

Paddock, who had no criminal record, killed himself either moments before or after SWAT police stormed his hotel room. The firearms were found inside his room and a hammer-like device that he used to smash the room's window before opening fire, authorities said.

"My fellow Americans, we are joined today in sadness, shock and grief," US President Donald Trump said in his first address following the incident.

"Our unity cannot be shattered by evil, our bonds cannot be broken by violence and though we feel such great anger at the senseless murder of our fellow citizens, it is our love that defines us today and always will forever."

Paddock lived in Mesquite, Nevada with Ms Danely, a 62-year-old Australian citizen who lived on the Gold Coast for more than a decade.

She was initially described as a "person of interest" being sought by police. The Las Vegas Metropolitan Police Department said in a statement late on Monday evening that "detectives have made contact with her and do not believe she is involved with the shooting on the strip."

However, in an update on Tuesday morning, Las Vegas Metropolitan Police Department Sheriff Joe Lombardo said the gunman's companion was still a "person of interest" and she would be interviewed when she returned to the US.

Ms Danley was reportedly in the Philippines at the time of the shooting, although there have been conflicting reports about her location.

Relatives told the Washington Post and local media that she had been Paddock's girlfriend for some time and lived with him.

"She's a nice lady, I've met her three or four times," Paddock's brother, Eric, told local TV station WKMG from his home in Orlando, Florida. "She's got a Facebook page. She sends cookies to my mum."

Her LinkedIn profile said she was a "gambling and casino professional" who lived in Reno, Nevada and, between 2010 and 2013, worked as a "hostess" for the Atlantis Casino Resort Spa.

Facebook photos show her participating in a gambling tournament in Las Vegas in 2014.

Paddock's home in an over-55s residential community in Mesquite, Nevada, about an hour from Las Vegas, was searched by local police on Monday.

Paddock had 16 guns in his hotel room when his body was found by police, according to law enforcement authorities; another 18 were found at his home, they said, plus various explosives, including ammonium nitrate, a common fertiliser that can be turned into a bomb.

A Mesquite Police Department spokesman said Paddock had no previous interactions with police. He said Paddock lived in a quiet cul-de-sac and nothing of note was found in his home, although there were some guns and ammunition.

Eric Paddock told reporters outside his home that there was "absolutely no indication he could do something like this".

"It's like an asteroid fell out of the sky," his visibly shocked brother said. "He was just a guy. Something happened, he snapped or something. We know absolutely nothing, this is just, we are dumbfounded."

He said his brother had no political or religious affiliations and didn't own any machine guns when he moved to Mesquite a few years ago. Their deceased father, Benjamin Paddock, was on the FBI's Most Wanted List in 1961 for bank robberies, the FBI confirmed to CNN.

Las Vegas Metropolitan Police Department Sheriff Joe Lombardo said Paddock had no local, state or federal convictions and they had no indication of a motive.

Washington Post

Hours after the attack, Islamic State issued a message on their official Telegram channel claiming responsibility for the attack. The message said the shooter had converted to Islam months ago. However, the FBI said there was no indication of any links to foreign terror groups.

Two off-duty policemen attending the concert are believed to be among the dead.

"We believe there are a couple of off-duty officers attending the concert that may have died," Las Vegas Police Department Sheriff Joe Lombardo said.

Earlier reports of a second shooter and suspicious packages and explosions in other hotels were false, Sheriff Lombardo said.

Danita Cohen, a spokeswoman for the University Medical Centre, said the hospital received 106 patients, eight of whom went straight to operating theatres.

An Australian witness staying at the Mandalay Bay hotel told CNN that there was mayhem as the shots broke out.

"The initial gun shots were machine-gun fire," he said. "We knew it was chaos and mayhem because people were running out of everywhere."

He also described scenes of chaos when police came across a group of guests taking cover.

"We were tucked away, about 12 of us, in a carpark thing, and the Swat team came and said hands up," he said.

"We didn't know where the shooter is. We knew they were confused like we were. Police ushered us out".

Another witness said: "Police said run and we started running."

Mandalay Bay is located on the south end of the Vegas strip and across from McCarran International Airport.

Flights in out of McCarran Airport had been stopped.

Firearms can generally be carried openly throughout Nevada, though carrying a loaded rifle or shotgun inside a vehicle or on or along a public road is illegal. Although North Las Vegas has an ordinance prohibiting open carry at county parks or in vehicles within city limits, the ordinance does not supersede state law.

The Department of Foreign Affairs and Trade said in a short statement on Monday night that it was still trying to determine whether any Australians had been caught up in the incident.

"The Australian Consulate-General in Los Angeles is making urgent inquiries with local authorities to determine whether any Australians have been affected by a shooting incident at the Mandalay Bay Resort and Casino in Las Vegas," the spokeswoman said."If you have any concerns for the welfare of family and friends in the Las Vegas area, you should attempt to contact them directly."

Those unable to contact friends and family in Las Vegas are urged to contact the Department of Foreign Affairs and Trade's 24-hour Consular Emergency Centre on 1300 555 135, or +61 2 6261 3305.