WASHINGTON (AFP, REUTERS) - US President Donald Trump sent his "warmest condolences" Monday (Oct 2) to the victims and families of a shooting at a concert in Las Vegas which has killed at least 50 people.

"My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!" Trump wrote on Twitter in response to what is the deadliest mass shooting in modern US history.

White House spokesman Sarah Sanders said separately that the president had been briefed on "the horrific tragedy in Las Vegas" on Sunday night.

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

First lady Melania Trump tweeted her "heart and prayers" to the victims and those affected. Ivanka Trump, the President's eldest daughter and a top adviser, tweeted "our collective hearts are breaking for the victims and their families."

Hillary Clinton, Trump's Democratic Party rival in last year's election, swiftly called for gun control measures in the hours after the shooting.

She called out the National Rifle Association (NRA) for backing legislation that would make it easier for Americans to purchase gun silencers.

'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 former Democratic nominee for president said.

In Rome, Pope Francis called the killing a "senseless tragedy".

"Deeply saddened to learn of the shooting in Las Vegas, Pope Francis sends the assurance of his spiritual closeness to all those affected by this senseless tragedy," said a message sent to the Roman Catholic bishop of the Nevada city on Monday.

The shooting broke out on the final night of the three-day Route 91 Harvest festival, a sold-out event attended by thousands and featuring top acts such as Eric Church, Sam Hunt and Jason Aldean.

Aldean initially carried on playing when the first crackle of gunfire could be heard but then hurried off the stage once he realised that it was a shooting.

A shocked Aldean told his fans via Instagram that he and his band was safe. "Tonight has been beyond horrific," the singer wrote. "I still don't know what to say ... My Thoughts and prayers go out to everyone involved tonight.

"It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."