The man accused of hiding out with a gun near Donald Trump's Florida golf course in an apparent bid to kill the former president wrote a letter months earlier describing an "assassination attempt" and offering a bounty on Trump's life, US prosecutors say.

Ryan Routh, 58, has been charged with two gun crimes after he allegedly pointed a rifle through the tree line on September 15 while the Republican presidential candidate was playing golf at his course in West Palm Beach, according to a criminal complaint. He has not yet entered a plea.

Mr Routh is due to appear at a hearing scheduled to begin on Monday local time in which prosecutors will ask a judge to keep him in jail until his trial.

In a court filing released before the hearing, prosecutors said that several months prior to the incident, Mr Routh dropped off a handwritten letter addressed to "the world" that offered a bounty on Trump.

"This was an assassination attempt on Donald Trump but I failed you," the suspect wrote, according to the filing. "I will offer $150,000 to whomever can complete the job."

The letter was found in a box handed over by an unidentified civilian witness that also included ammunition, a metal pipe and four phones, prosecutors said.

Reuters