US President Donald Trump visited a hospital and a police base in Las Vegas on Wednesday, offering prayers and condolences to the victims of Sunday night's shooting massacre, along with the nation's thanks to first responders and doctors who rushed to save lives.

"America is truly a nation in mourning," the president declared, days after gunman Stephen Paddock, 64, who was in a 32nd-floor hotel room, opened fire on the crowd at an outdoor country music festival below.

Trump spoke of the families who "tonight will go to bed in a world that is suddenly empty".

"Our souls are stricken with grief for every American who lost a husband or a wife, a mother or a father, a son or a daughter," he told them. "We know that your sorrow feels endless. We stand together to help you carry your pain. "

Paddock killed 58 people and wounded more than 500 others before killing himself in his room at the Mandalay Bay Resort and Casino, authorities said. The Associated Press previously reported that 59 victims were killed, but has received revised information from the Clark County coroner.

Paddock's girlfriend said Wednesday that she had no inkling of the massacre he was plotting when he sent her on a trip abroad to see her family.

Marilou Danley issued the statement after returning from her native Philippines and being questioned for much of the day by FBI agents still trying to figure out what drove Paddock to open fire on 22,000 fans at a country music festival from his 32nd-floor hotel suite.

"He never said anything to me or took any action that I was aware of that I understood in any way to be a warning that something horrible like this was going to happen," Danley said in a statement read by her lawyer outside FBI headquarters in Los Angeles.

Danley, 62, who was overseas for more than two weeks, said she was initially pleased when Paddock wired her money in the Philippines to buy a house for her family. But she later feared it was a way to break up with her.

The high-stakes gambler and real estate investor specifically requested an upper-floor room with a view of the music festival when he checked in last Thursday, according to a person who has seen hotel records turned over to investigators.

The room, which goes for $590, was given to Paddock free because he was a good customer who wagered tens of thousands of dollars each time he visited the casino, the person said.