A gunman on the 32nd floor of a Las Vegas Strip casino unleashed a shower of bullets on an outdoor country music festival below, killing at least 58 people and wounding more than 515, officials said on Monday. It was the worst mass shooting in the modern United States (US) history.

Country music star Jason Aldean was performing Sunday night at the end of the three-day Route 91 Harvest Festival when the gunman opened fire across the street from inside the Mandalay Bay Hotel and Casino.

SWAT teams quickly descended on the concert and the casino, and officers used explosives to get into the hotel room where the suspect was inside, authorities said.

The gunman died at the scene and was identified by Clark County Sheriff Joseph Lombardo as Stephen Paddock. A motive has not been ascertained as yet. Police are also looking a woman, identified as Marilou Danley, for questioning.

The gunman, identified as Stephen Craig Paddock, a 64-year-old retiree from Mesquite, Nevada, had as many as 10 guns with him, including rifles, the authorities said.

Asked about the motive for the attack, Sheriff Joseph Lombardo said: “I can't get into the mind of a psychopath at this point.” The sheriff said a check of federal and state databases showed Paddock was not on law enforcement authorities' radar before the bloodbath.

Aaron Rouse, the FBI agent in charge in Las Vegas, said investigators saw no immediate evidence connecting the attack to an international terror organization, despite a claim of responsibility from the militant Islamic State (IS) group.

“We are looking at excess of 50 individuals dead and of 200 individuals injured at this point,” Las Vegas Metro Police Sheriff Joseph Lombardo said at a press conference after the shooting.

US President Donald Trump took to Twitter more than 4 hours after the shooting started to offer his condolences.

The IS group has claimed responsibility for the mass shooting in Las Vegas, saying that the perpetrator was “a soldier” who had converted to Islam months ago, without providing any evidence to support the claim.

The group released two statements on its Amaq news agency on Monday, hours after the shooting.

It did not name the suspected shooter but said he had “executed the operation in response to calls to target countries of the coalition” batting the group in Iraq and Syria.

IS often claims attacks by individuals inspired by its message but with no known links to the group.

Gunfire starts mid-concert

Aldean was in the middle of a song when the shots came rapidly.

Video of the shooting then showed Aldean stopping and the crowd getting quiet as if they were unsure of what had just happened.

The gunman paused and then fired another volley of muzzle flashes from the gold glass 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.

Pictures taken at the scene showed revellers cowering in the street or running in terror, as well what appeared to be several bodies lying prone on the ground.

Kodiak Yazzie, 36, said the music stopped temporarily when the first shots began and the tune even started up again 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.”

Thousands in the crowd fled as the bullets ran rampant.

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, 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 roommate Marilou Danley, 62. Lombardo said they believe this was a “lone wolf” attack.

“It's a devastating time,” Lombardo said.

Police 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.

'Beyond horrific'

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.

Jose Baggett, 31, of Las Vegas, said he and a friend were in the lobby of the Luxor hotel-casino directly north of the festival when people began to run, almost like in a stampede.

He said people were crying and as he and his friend started walking away minutes later, they encountered police checkpoints where officers were carrying shotguns and assault rifles.

“There were armored personnel vehicles, SWAT vehicles, ambulances, and at least a half-mile of police cars,” Baggett said.

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 and said 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,” Aldean said.

The shooting at the sold-out Route 91 Harvest festival 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.

Sunday's shooting came more than four months after a suicide bombing at an Ariana Grande concert in Manchester, England, that killed 22 people.

Almost 90 people were killed by gunmen inspired by the militant Islamic State group at the Bataclan concert hall in Paris during a performance by Eagles of Death Metal in November 2015.