Donald Trump tweeted his reaction to the attack by the Mandalay Bay resort, in which at least 50 people were killed and 200 injured.
After a White House official confirmed he had been briefed, the US president took to Twitter to express his sympathies to the victims.
He said: “My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!”
No official statement from Mr Trump has yet been issued.
Mr Trump’s daughter Ivanka also wrote a Twitter tribute.
She said: “America woke up this morning to the horrific news of a devastating mass shooting in Las Vegas.
“Our collective hearts are breaking for the victims and their families.”
The death toll, which police emphasized was preliminary and tentative, would make the attack the deadliest mass shooting in U.S. history, eclipsing last year’s massacre of 49 people at an Orlando night club.
Thousands of panicked people fled the scene, in some cases trampling one another as law enforcement officers scrambled to locate and kill the gunman.
Shocked concertgoers, some with blood on their clothes, wandered the streets after the attack.
Police identified the gunman as area resident Stephen Paddock, 64, and said they had no information yet about his motive.
He was not believed to be connected to any militant group, Clark County Sheriff Joseph Lombardo told reporters.
“We have no idea what his belief system was,” Lombardo said. “We’ve located numerous firearms within the room that he occupied.”
This is an ongoing story, more to follow…