US Republican presidential nominee Donald Trump was the target of "what appears to be an attempted assassination" on Sunday afternoon at his golf club in West Palm Beach, Florida, the FBI said — just two months after he survived a previous attempt on his life.

Both incidents highlight the challenges of keeping presidential candidates safe in a hotly contested and polarized campaign with just more than seven weeks to go before the Nov 5 election.

Trump sent an email to supporters soon after the incident saying he was "safe and well".

Palm Beach County Sheriff Ric Bradshaw said Secret Service agents fired at the suspect, who was pointing an AK-47-style rifle with a scope as Trump was on the golf course.

Bradshaw said the gunman also had two backpacks and a GoPro camera and was about 365 to 460 meters away, hiding in shrubbery — with the muzzle of the weapon visible — while Trump played on a nearby hole. The suspect dropped the weapon and fled in an SUV.

The golf course was partially closed for Trump as he played, and agents were a few holes ahead of him when they noticed a man with the firearm, Bradshaw said.

CNN, Fox News and The New York Times identified the suspect as Ryan Wesley Routh, 58, from Hawaii, citing unidentified law enforcement officials. The FBI declined to comment.

It was not clear if or how the suspect knew Trump was playing golf at the time, or what his motive was, but the attempted attack is sure to raise new questions about the level of protection he is given.

CNN and CBS reported Routh was a self-employed affordable housing builder in Hawaii who had an arrest record spanning decades and regularly posted on politics and current events, including sometimes criticizing Trump.

Apprehending suspect

In neighboring Martin County, Sheriff William Snyder said the suspect was apprehended within minutes of the FBI, Secret Service and Palm Beach County Sheriff's Office putting out an urgent alert.

Snyder told WPTV that the suspect "was not armed when we took him out of the car". The man had a calm, flat demeanor and showed little emotion when he was stopped by police, Snyder said, adding he did not question why he was being pulled over.

"He never asked, 'What is this about?' Obviously, law enforcement with long rifles, blue lights, a lot going on. He never questioned it," Snyder said.

Fox News host Sean Hannity said on the air that he spoke with Trump and the former president's golf partner, Steve Witkoff, after the incident.

They told Hannity they had been on the fifth hole and about to go to putt when they heard "pop pop, pop pop". Within seconds, Secret Service agents "pounced on" Trump and "covered him", Witkoff said.

Trump returned to Florida on the weekend after a West Coast campaign trip. His Sunday schedule had not been made public by the campaign. Trump often spends the morning golfing before having lunch at the club.

He has had increased security since the assassination attempt in July in Butler, Pennsylvania. When he has stayed at Trump Tower in New York, a line of dump trucks have been parked outside the building. At outdoor rallies now, he speaks behind bulletproof glass.

The White House said President Joe Biden and Vice-President Kamala Harris, the Democratic presidential nominee, were briefed and would be kept updated on the investigation. It said the president was "relieved" that Trump is safe.

In a statement, Harris said she was "glad" Trump was safe, adding, "Violence has no place in America."

The leaders of the bipartisan task force in Congress, which has been investigating the earlier assassination attempt, said on Sunday it is monitoring the situation and has requested a briefing by the Secret Service.

During the July 13 rally in Pennsylvania, Trump was fired upon, with a bullet grazing his ear. A rallygoer was killed.

The shooting forced Kimberly Cheatle to resign as Secret Service director under bipartisan pressure from Congress.

The gunman, identified as Thomas Crooks, 20, was shot and killed by a Secret Service sniper.

Agencies contributed to this story.