West Palm Beach, Florida - A man suspected of plotting to assassinate former US president Donald Trump was charged with federal gun crimes on Monday.

Ryan Wesley Routh (58) was charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number at his initial court appearance.

Routh, who was arrested on Sunday after an alleged abortive bid to shoot the former president at his West Palm Beach golf course, is expected to face further charges at a later arraignment.

Routh appeared calm during the brief court hearing, answering "yes" in a soft voice to several questions from District Judge Ryon McCabe of the Southern District of Florida.

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Routh was arrested on Sunday after a US Secret Service agent noticed the barrel of a rifle pointing out of shrubbery at the Trump International Golf Course.

Agents, who were deployed in a moving security bubble extending about one or two holes ahead of the former president, "engaged" an individual, the US Secret Service's Rafael Barros said.

Trump was unharmed.

Police said Routh fled in a vehicle and was arrested shortly after.

Sheriff Ric Bradshaw said police recovered an "AK-47-style" rifle equipped with a scope, two backpacks, and a GoPro camera from the scene.

Routh is reported to be a self-employed affordable housing builder based in Hawaii, with an arrest record spanning decades.

He reportedly regularly posted about politics and current events on social media, including criticism of Trump.