President Donald Trump condemned the massacre of at least 59 Las Vegas concert goers as an "act of pure evil" on Monday, but refrained from addressing calls for gun control or the motives for the worst mass shooting in recent US history.

Delivering televised remarks, Trump tried to offer consolation and called for unity — an act that has become a grim rite of passage for modern US presidents.

Barack Obama wept as he tried to soothe the nation after the 2012 Sandy Hook primary school shooting, George W. Bush told Americans that the "nation grieves" after a similar outrage at Virginia Tech university.

Trump ordered that flags be flown at half-staff until sunset on Friday, offered prayers for the victims and announced he would visit Las Vegas on Wednesday.

Later, he led White House staff on the South Lawn for a moment of silence.

"In moments of tragedy and horror, America comes together as one — and it always has," Trump said.

Police have identified the gunman behind the Sunday night massacre — which injured more than 500 people — as a 64-year-old former accountant named Stephen Craig Paddock, who killed himself before a SWAT team breached his 32nd floor hotel room.

Investigators recovered at least 16 guns, including assault rifles, from Paddock's room at the Mandalay Bay, and another 18 firearms along with bomb-making materials at one of his two homes.

Officials have reacted cautiously to an Islamic State group claim that Paddock was a "soldier of the caliphate" but while his motive remained unclear, the shooting instantly rekindled the divisive national debate on gun control.

'Right to bear arms'

White House Press Secretary Sarah Huckabee Sanders said that now was not the time for politics, a tactic frequently used by gun advocates to diffuse public outrage.

"There's a time and place for a political debate, but now is the time to unite as a country," said Sanders.

Trump insisted, "our unity cannot be shattered by evil. Our bonds cannot be broken by violence."

But in the immediate aftermath of the shooting, that unity was difficult to find.

Trump's vanquished election rival Hillary Clinton hit out at the gun manufacturers lobby — the National Rifle Association — which has backed a congressional push to make it easier to obtain a gun silencer.

"The crowd fled at the sound of gunshots. Imagine the deaths if the shooter had a silencer," tweeted Clinton, whose Democratic Party has tried in vain to introduce lasting gun control measures.

"Our grief isn't enough. We can and must put politics aside, stand up to the NRA, and work together to try to stop this from happening again."

There have already been over 270 mass shootings in the United States this year alone, according to www.massshootingtracker.org, although the exact definition is contested.

Gun violence accounts for more than 33,000 deaths each year in the United States, and according to the latest Gallup poll, 55 percent of American voters would like to see stricter rules for buying guns.

But the issue is highly sensitive and Trump's own views have changed markedly over his years in public life.

After the Sandy Hook Elementary School shooting, where 20 six and seven-year old children and six adults were mowed down by a disturbed 20-year-old, Trump appeared to favor stricter rules.

Back then, his predecessor Obama — who often called Sandy Hook the worst moment of his eight year presidency — called for the deadlock to be broken and for Congress to act.

At that time Trump tweeted: "President Obama spoke for me and every American in his remarks in #Newtown Connecticut."

But since then Trump — whose White House bid was endorsed by the NRA — has positioned himself as a defender of the constitutional "right to keep and bear arms."

The NRA donated an estimated $30 million to Trump's campaign in 2016, according to the Center for Responsive Politics.

US Senator Chris Murphy, who was the congressman for Sandy Hook, renewed a call for action in the wake of the Las Vegas attack.

"This must stop. It is positively infuriating that my colleagues in Congress are so afraid of the gun industry that they pretend there aren't public policy responses to this epidemic," he said.