A man suspected of attempting to assassinate Donald Trump appeared in federal court on Monday, where he was charged with two gun-related crimes a day after being spotted with a rifle hiding in the bushes at the former US president's golf course in Florida.

More charges appear likely, but the initial counts – possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number, according to the US Justice Department – will allow authorities to keep him in custody as the investigation continues.

On Sunday, the US Secret Service opened fire after an agent saw a rifle barrel poking out of the bushes at Trump's golf course in West Palm Beach, a few hundred metres away from where the former president was playing. The gunman fled by car, leaving behind two backpacks and his weapon. A suspect, later identified as Ryan Routh, 58, was later arrested.

Routh wore dark prison scrubs and his hands and feet were shackled during an appearance on Monday morning in a federal courtroom in West Palm Beach, CNN reported.

Trump, the Republican presidential candidate in the November 5 election, was unharmed but the incident raised fresh questions about how an armed suspect was able to get so close to him, just two months after another gunman fired at Trump during a July 13 rally in Butler, Pennsylvania, grazing his ear with a bullet.

The Secret Service, which protects presidential candidates, "needs more help," including possibly more personnel, President Joe Biden told reporters on Monday morning, adding: "Thank God the president's OK."

The agency came under intense scrutiny after the earlier attempt on Trump's life, which led to the resignation of Director Kimberly Cheatle. The service bolstered Trump's security detail following the July 13 attack, in which the gunman was shot dead by responding agents.

House of Representatives Speaker Mike Johnson, who convened a bipartisan task force to investigate after the first assassination attempt, said in a Fox News interview that Congress would also examine the latest incident.

"We need accountability," said Johnson who also called for more resources to protect Trump. "We must demand that this job is being done." (Reuters)