Delivering televised remarks from the White House, Trump ordered that flags be flown at half-staff until sunset Friday, offered prayers for the victims and announced he would visit Las Vegas on Wednesday.
Trump did not address an IS propaganda agency's unverified claim that one of its "soldiers" who had "converted to Islam" months ago was responsible.
White House officials also refused to comment on that claim "at this point" saying the local authorities' investigation was "ongoing." The FBI says it has so far found "no connection" between international terrorist groups and the Vegas attack.
Calling for unity, Trump instead tried to console the nation -- an act that has become a grim rite of passage for modern US presidents as each mass shooting rekindles the divisive national debate on gun control.
"In moments of tragedy and horror, America comes together as one -- and it always has," Trump said.
"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."
But in the immediate aftermath of the shooting, that unity was difficult to find.
- 'Right to bear arms' -
Police have identified the heavily armed gunman behind the Sunday night massacre in Las Vegas -- the deadliest mass shooting in modern US history which left more than 500 injured -- as a 64-year-old Nevada resident named Stephen Craig Paddock.
Paddock apparently killed himself before a SWAT team breached his hotel room overlooking the concert venue, recovering at least eight rifles.
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 despite the national scourge of mass shootings.
"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."
The issue of gun control is highly sensitive in the United States and Trump's own views on the issue have changed markedly over his years in public life.
After the 2012 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, president Barack Obama -- who often called Sandy Hook the worst moment of his eight year presidency and recalls even his Secret Service detail in tears -- 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 eagerly endorsed by the NRA -- has positioned himself as a staunch defender of the constitutional "right to keep and bear arms."
US Senator Chris Murphy, who was the congressman for Sandy Hook, renewed a call for action in the wake ofthe Las Vegas attack.
"Nowhere but America do horrific large-scale mass shootings happen with this degree of regularity," he said.
"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."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)