President Trump called the Las Vegas attack "an act of pure evil" and called for national unity in the wake of the deadliest mass shooting in modern U.S. history.

"Hundreds of our fellow citizens are now mourning the sudden loss” of a parent, child, friend or other family member, Trump said at the White House on Monday. "We cannot fathom their pain, we cannot imagine their loss."

Trump announced that he would visit Las Vegas on Wednesday, and said the FBI and Department of Homeland Security are working closely with local authorities. He praised the local police department and first responders for their quick response to the attack.

On Sunday night, a gunman opened fire from a hotel room above a packed, outdoor concert in Las Vegas, an attack that killed at least 50 and wounded at least 400 more.

White House Press Secretary Sarah Sanders said Trump was briefed early Monday, and "we are monitoring the situation closely and offer our full support to state and local officials. All of those affected are in our thoughts and prayers.”

It is the first mass shooting under Trump's watch as his ability to console the nation is already being tested by his administration's response to hurricanes in Texas, Florida and Puerto Rico.

There were at least 14 mass shootings during President Barack Obama's two terms, a source of significant frustration for the president which he described in 2015 as having "no parallel anywhere else in the world."

Vice President Pence also extended sympathies to the victims of the Las Vegas shooting via a series of tweets.

"To the courageous first responders, thank you for your acts of bravery," Pence said in one tweet.

Clark County Sheriff Joe Lombardo said the shooter, identified as Stephen Paddock, 64, was found dead by officers who stormed his 32nd-floor room at the Mandalay Bay Resort and Casino.

The shooting is also likely to lead to renewed calls by congressional Democrats for increased gun restrictions, including universal background checks. Yet with Republicans in control of both chambers and the White House those efforts are unlikely to gain traction.

Trump is still scheduled to travel Tuesday to Puerto Rico to review hurricane recovery efforts.