President Donald Trump has arrived in Las Vegas to visit a city shaken by the deadliest shooting spree in modern US history, praising the heroes who responded to the “worst of humanity” with the best of humanity.

The president and First Lady, Melania Trump, visited Clark County Sheriff Joseph Lombardo after landing on Wednesday, then departed for the local hospital.

Mr Trump said the US will “endure the pain together and overcome it together as Americans”, thanking the heroes for their bravery during this difficult time.

“When the worst of humanity strikes, and strike it did, the best of humanity responds,” Mr Trump said.

He added that it made him proud to be an American after visiting some of the hospitalised victims of the shooting.

He refused to respond to questions regarding US gun laws: “We’re not going to talk about that today.”

Stephen Paddock, 64, opened fire on a country music concert earlier this week, killing at least 58 and wounding more than 500 people.

Paddock ended his own life when a police SWAT team closed in on him in his Mandalay Bay Hotel suite.

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It’s the first time Mr Trump will deal directly with the aftermath of a major mass shooting.

“We’ll, it’s a very sad thing. We’re going to pay our respects and to see the police who have done really a fantastic job in a very short time,” Mr Trump told reporters at the White House.

“It’s a very, very sad day for me, personally.”

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Paddock, a retiree with no criminal record, was identified as the gunman in Sunday night’s attack, spraying bullets at an open-air concert from the window of his suite in a high-rise hotel.

Mr Trump’s motorcade passed the hotel from the freeway during the drive to the hospital.

Paddock killed himself as police closed in.

He had no known history of mental illness and no outward signs of social disaffection, political discontent or extremist ideology, police said.

“We are not there yet,” FBI Deputy Director Andrew McCabe said on Wednesday, speaking at the Cambridge Cyber Summit in Boston. “We have a lot to do.”

However, Paddock appeared to be “descending into madness” in the months before the shooting, ABC News reported on Wednesday, citing an unnamed person briefed on the investigation.

Donald Trump and Melania meet with Las Vegas first responders and civilians who helped during the mass shooting. Photo: AP

Paddock had significant weight loss, an increasingly slovenly appearance and was obsessed with his girlfriend’s former husband, ABC said.

In June he was prescribed the anti-anxiety drug diazepam, commonly known as Valium, which can lead to aggressive behaviour, the Las Vegas Review-Journal reported, citing records from the Nevada Prescription Monitoring Program.

Reuters was not able immediately to confirm the two reports.

Mr Trump said “a lot more” was being learned about Paddock.

“That’ll be announced at the appropriate time,” he said.

On Tuesday, he called Paddock sick and demented.

A Reuters image capturing Mr Trump and his wife flying out of Las Vegas with the Mandalay Bay hotel in the foreground has since gone viral.

“Wow. This presidency in one picture,” one Twitter user wrote.

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Shooting prompts gun law debate

Mr Trump has had mixed success in the “consoler-in-chief” role that is periodically expected of US presidents.

He inflamed racial tensions after a white-supremacist rally in Charlottesville, Virginia, and he has struggled to strike the right tone in response to hurricane devastation in Puerto Rico.

The Las Vegas shooting has reignited a debate in the United States about whether more gun control legislation might have prevented what happened.

Mr Trump, a Republican who firmly aligned himself with gun rights advocates during last year’s presidential campaign, was asked on Tuesday whether it was time to debate gun control measures.

He responded, “Perhaps that will come. But that’s not for now.”

He has called the massacre “an act of pure evil”.

-with AAP