US President Barack Obama has dismissed as "ridiculous" charges by Republican candidate Donald Trump that the US presidential election is going to be rigged against him.

"Of course the elections will not be rigged - what does that mean?" Obama said at a press conference Thursday after Trump had insinuated in recent days that the process was "rigged."

"If Mr Trump is suggesting there is a conspiracy theory that is being propagated across the country ... that's ridiculous, and I don't think anyone would take that seriously," Obama said.

Voting is overseen by local and state officials, but national officials take seriously the obligation to protect the voting process, Obama said during a press conference following meetings at the Pentagon defence headquarters.

He said it was unusual for someone to say they had been cheated of a win before the competition was even over.

"Go out there and try to win the election," Obama said.

Trump said this week that he feared November elections would be rigged, echoing statements he made during the party primaries.

Obama maintained that Trump will receive classified security briefings, a standard procedure for major-party presidential nominees.

After becoming their parties' nominees last month, Trump and Democratic candidate Hillary Clinton are both entitled to security briefings, which are meant to prepare them for office.

Obama also declined to weigh in on whether Trump should be given access to the country's nuclear codes. Clinton has repeatedly used concern about his temperament to argue he should not be put in charge of nuclear weapons or be made commander in chief of the military, and a unsubstantiated reports that Trump had asked why the US should not use the weapons had increased concerns.

Obama said he had made clear earlier in the week that he believes Trump is "unfit" for the office and noted "this is serious business."

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