US President Donald Trump has denounced the mass shooting at a Las Vegas music festival as an act of pure evil, saying Americans are joining together in sadness, shock and grief.

In brief remarks from the White House on Monday, Trump praised first responders, including the Las Vegas Metropolitan Police Department, for their courageous efforts. The president also said he would visit Las Vegas on Wednesday.

The gun attack on the Mandalay Bay Resort and Casino killed at least 58 people and injured more than 500 others. Police said the suspect, identified as 64-year-old Stephen Paddock from Mesquite, shot himself to death.

Last night a gunman opened fire on a large crowd at a country music concert in Las Vegas, Nevada. He brutally murdered more than 50 people and was an act of pure evil, Trump said in his televised address.

We cannot fathom their pain, we cannot imagine their loss, he said of those who lost loved ones in the incident.

The president said the Federal Bureau of Investigation (FBI) and Department of Homeland Security were working with local authorities on the investigation.

A body lies under a sheet as fire and rescue personnel gather at the intersection of Las Vegas Boulevard and Tropicana Ave after the mass shooting. (Photo by AFP)

Trump also called for unity in the aftermath of the shooting, the largest in US history. Our unity cannot be shattered by evil, our bonds cannot be broken by violence.

In times such as these I know we are searching for some kind of meaning in the chaos, some kind of light in the darkness, he said.

The answers do not come easy. But we can take solace knowing that even the darkest space can be brightened by a single light, and even the most terrible despair can be illuminated by a single ray of hope.

Trump had been briefed earlier in the morning on the shooting and offered, in a tweet, his “warmest condolences and sympathies to the victims and families of the massacre.

Shortly after the US president delivered his statement, the White House released a proclamation ordering flags be flown at half-staff on federal buildings.