Former United States president Donald Trump survived a shooting, in which he reportedly was grazed by a bullet, shortly after the start of his rally in Butler, Pennsylvania, on Saturday evening.

In the moments after the shooting, Trump was swarmed and shielded by Secret Service agents. Trump's campaign later said he was "doing well" and appeared to have suffered no major injury.

China is following the incident, the Foreign Ministry said on Sunday. President Xi Jinping has expressed sympathies to Trump, the ministry said in a statement.

Early on Sunday, the FBI identified 20-year-old Thomas Matthew Crooks of Bethel Park, Pennsylvania, as the "subject involved" in what it termed an assassination attempt.

Earlier, the Secret Service said in a statement that the shooter was dead, one person attending the rally was killed and two other spectators were injured.

Law enforcement officials told reporters they had not yet identified the motive for the attack.

Around 6:15 pm local time, Trump reflexively reached for his right ear after several popping sounds, as Secret Service agents yelled "Get down! Get down!"

Trump, 78, quickly ducked and was swarmed by security agents, according to a widely circulated video that was taken close to the front of the stage.

Trump could be heard yelling "Fight! Fight! Fight" to the crowd in the immediate aftermath, and said, "Let me get my shoes."

Trump issued a statement on his social media platform Truth Social late on Saturday, thanking law enforcement and expressing condolences to the families of the person who was killed at the rally and the injured.

The attack apparently came from a sniper positioned on a building a few hundred yards from Trump — outside the security cordon, according to law enforcement sources cited by the New York Post.

"President Trump thanks law enforcement and first responders for their quick action during this heinous act," Trump campaign spokesman Steven Cheung said in a statement.

The attack was the first shooting targeting a US president or major party candidate since the 1981 attempted assassination of Republican president Ronald Reagan, Reuters reported.

The shooting occurred less than four months before the Nov 5 election, in which Trump, a Republican, is expected to face an election rematch with President Joe Biden, a Democrat.

Mo Jingxi in Beijing and agencies contributed to this story.