At least 58 people have been killed and another 515 were hurt after a gunman opened fire on a Las Vegas outdoor country music festival in the deadliest mass shooting in modern US history.

SWAT teams who stormed the gunman's room on the 32nd floor of the Mandalay Bay Hotel and Casino skyscraper found he had killed himself after shooting at revellers enjoying the Route 91 Harvest Festival.

The gunman, identified as Stephen Craig Paddock, 64, of Mesquite, Nevada, had as many as 10 guns with him, including rifles, authorities said.

There was no word on a motive for the attack, but a law enforcement official said there was no immediate indication that the massacre was connected to international terrorism.

The Islamic State group claimed responsibility for the attack and said the gunman was "a soldier" who had converted to Islam months ago, but it provided no evidence to back up the claim.

Country music star Jason Aldean was performing on Sunday night at the end of the three-day festival in front of a crowd of more than 22,000 when the gunman opened fire from inside the 44-floor Mandalay Bay Hotel and Casino across the street on Las Vegas Strip.

Paddock had checked into the hotel room on Thursday, authorities said. Police said he was a retiree with no criminal record in the Nevada county where he lived.

The gunman's brother, Eric Paddock, who lives in Florida, told the Orlando Sentinel: "We are completely dumbfounded. We can't understand what happened."

Prime Minister Theresa May wrote on Twitter: "The UK's thoughts are with the victims and the emergency services responding to the appalling attack in Las Vegas."

Foreign Secretary Boris Johnson said he was "horrified" by the attack, and that his thoughts were with those caught up in it.

The Foreign Office said it was in contact with authorities in Las Vegas and that it was "ready to help" any British people affected.

A spokeswoman said: "Any British people in Las Vegas should follow the advice of emergency services, and check the Foreign Office travel advice."

British tennis star Laura Robson was among those attending the concert.

She shared two pictures of herself at the event, wearing a cowboy hat with an American flag emblazoned on it.

Asked by a Twitter follower if she was all right, she wrote: "I'm okay. We were right there ... sounded like fireworks at first then everyone started running."

She added it had been a "scary" experience.

In an address to the country, US president Donald Trump called the attack "an act of pure evil" and added: "In moments of tragedy and horror, America comes together as one. And it always has."

Sheriff Joseph Lombardo said authorities believe it was a "lone wolf" attack, while the US Homeland Security Department said there was no "specific credible threat" involving other public venues in the US.

US attorney general Jeff Sessions offered the support of the FBI and other federal agencies, but noted that the investigation is being led by the sheriff in Las Vegas - signalling that the shooting was not believed to be an act of international terrorism.

Las Vegas authorities called for blood donations and set up a hotline to report missing people and speed the identification of the dead and wounded. They also opened a "family reunification centre" for people to find loved ones.

Harrowing video footage of the shooting shows Jason Aldean stopping his performance on stage after an initial volley of shots could be heard.

After a pause, the gunman fired another volley, with the muzzle flashes visible from the casino as victims fell to the ground, while others fled in panic.

Police shut down busy Las Vegas Boulevard, and federal and state authorities converged on the scene. Interstate 15 was briefly closed, and flights at McCarran International Airport were suspended.

Hospital emergency rooms were jammed with the wounded. Ruben Kihuen, a Democrat whose congressional district includes a portion of Las Vegas, visited a hospital were some of the victims were taken and said: "Literally, every single bed was being used, every single hallway was being used. Every single person there was trying to save a life."

The dead included at least three off-duty police officers from various departments who were attending the concert, authorities said. Two on-duty officers were wounded, one critically, police said.

Nearly every inch of the Las Vegas Strip is under video surveillance, much of it set up by the casinos to monitor their properties, which could yield a wealth of material for investigators as they try to piece together the attack.

Hours after the shooting, Aldean posted on Instagram that he and his crew were safe and that the shooting was "beyond horrific".

"It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night," the country star said.

Before Sunday's massacre, the deadliest mass shooting in modern US history took place in June 2016, when a gunman who professed support for Muslim extremist groups opened fire at a gay nightclub in Orlando, Florida, killing 49 people.

Evening Times:

Gunman: Stephen Paddock

Three off-duty British soldiers rushed to the aid of the injured, a spokesman for the Ministry of Defence confirmed.

The soldiers from 1st The Queen's Dragoon Guards, known as the Welsh Cavalry, were on leave in Las Vegas after a desert training exercise in Nevada.

The spokesman said: "We can confirm that a number of serving personnel from 1st The Queen's Dragoon Guards provided assistance to the wounded following the heinous shooting in Las Vegas.

"Our thoughts go out to those affected by this terrible act."