US authorities have arrested a Florida man and plan to prosecute him over alleged threats to kill Democratic White House hopeful Barack Obama and President George W Bush, officials say.

Raymond Hunter Geisel, 22, was interviewed on August 1 by agents of the US Secret Service's Miami field office and arrested a day later, said US Secret Service spokesman Ed Donovan.

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According to a Secret Service affidavit, Geisel told a fellow student in a bail bondsman class "that nigger, if he gets elected, I'll assassinate him myself" - an apparent reference to Obama.

Another student in the class told a Secret Service agent that she had heard Geisel say that "that he hated George W Bush and that he wanted to put a bullet in the President's head," according to the affidavit.

A search of Geisel's car and home yielded a machete, a loaded nine millimetre handgun, a combat-style hatchet, knives, 40 rounds of 9mm armour-piercing handgun ammunition, two canisters of tear gas, and other items, according to the document.

During his August 1 interview, Geisel denied making threats against Obama or Bush but "did state, however, that if he wanted to kill Senator Obama he simply would shoot him with a sniper rifle, but then he claimed that he was just joking," according to the affidavit.

Later, he signed a statement that "'I Ray Geisel did not say I was going to kill any President or Senter Obama.' (sic)," according to the affidavit.

Geisel will face prosecution under a US law forbidding making threats against the president or a major candidate for the office of president, the affidavit said.

Asked how many such threats occur each year, Donovan said US Secret Service policy was not to disclose a precise number but said that "many, many threats are investigated each year."