AT LEAST 50 people have been killed and more than 200 others were injured after a gunman opened fire on an outdoor concert in Las Vegas in the worst mass shooting in modern US history.

The gunman, perched on the 32nd floor of the Mandalay Bay Hotel and Casino on Las Vegas Strip, opened fire on concert-goers at the Route 91 Harvest Festival during a set by country music star Jason Aldean, sparking panic among thousands of people who screamed and ran for their lives.

Swat teams quickly descended on the concert and the casino, and officers used explosives to get into the hotel room and confront the suspect.

The gunman, who died at the scene, was identified as Las Vegas resident Stephen Paddock.

A motive is not immediately clear.

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

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: "Our sympathies are with all those affected by the horrific shooting in Las Vegas.

"Details about the incident are still emerging and we are in contact with Las Vegas emergency services. We are ready to help any British people affected.

"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.

Video of the shooting showed Aldean stopping mid-song after gunfire could be heard.

The gunman then fired another volley, with muzzle flashes visible at the casino as more victims fell to the ground, while others fled in panic.

Some said they hid behind concession stands and other crawled under parked cars.

Witness Kodiak Yazzie, 36, said the music stopped temporarily when the first shots began. He said the song resumed, before the second round of pops sent the performers ducking for cover and fleeing the stage.

"It was the craziest stuff I've ever seen in my entire life," Yazzie said.

"You could hear that the noise was coming from west of us, from Mandalay Bay. You could see a flash- flash- flash- flash."

Monique Dumas from British Columbia, Canada, said she was at the concert, six rows from the front of the stage, when she thought she heard a bottle breaking, and then a burst of popping sounds that may have been fireworks.

She said as she made her way out that it was "organised chaos" as everyone fled.

"It took four to five minutes and all that time there was gunfire."

In addition to Paddock, police said they located a woman who may have been his room-mate - Marilou Danley, 62.

Officers shut down the usually busy Las Vegas Boulevard and authorities across the state and federal ranks converged onto the scene as dozens of ambulances ferried those struck by gunfire.

Nearby Interstate 15 and flights at McCarran International Airport were also halted.

Hospital emergency rooms were jammed with victims delivered by ambulance. Others loaded the wounded into their cars and drove them to hospitals.

Among those killed were two off-duty police officers who were attending the concert and two other on-duty officers were wounded, including one in critical condition, police said.

Hours after the shooting, Aldean posted on Instagram that he and his crew were safe, describing the shooting as "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," Aldean said.

The shooting is the worst mass shooting in modern US history.

Forty-nine people were killed when a gunman opened fire at the Pulse nightclub in Orlando in June 2016.

US president Donald Trump extended his condolences to the victims.

In a tweet, Trump offered "My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!"

White House spokeswoman Sarah Huckabee Sanders said Trump had been "briefed on the horrific tragedy in Las Vegas".

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