US President Donald Trump sent his "warmest condolences" Monday 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