BERLIN (AFP) - German Foreign Minister Frank-Walter Steinmeier believes Donald Trump becoming US president would be a "frightening" prospect for the world, his spokesman said on Wednesday (Aug 10).

Steinmeier "is not neutral" on whether the Republican candidate is fit to occupy the Oval Office, the spokesman, Sawsan Chebli, told reporters.

"He is of the opinion that it is frightening, if you follow Trump's remarks, what could become of this world if Trump actually became president," she said.

"The foreign minister is calling attention to that and that is his right."

Chancellor Angela Merkel's spokesman told the same news conference that she would stand by her policy of not commenting on the US election campaign.

Steinmeier, a Social Democrat, had already compared Trump last week to a "hate preacher", saying he had much in common with "fear-mongers" in Germany's right-wing populist AfD party as well as advocates of Britain's exit from the EU.

French President Francois Hollande said Trump makes "you want to retch" following the Republican's "hurtful and humiliating comments" against the Muslim parents of a slain US soldier who criticised the candidate.

Trump's bid to take the White House is reeling from a series of self-inflicted scandals after he disparaged Muslims, babies, firefighters and the military, prompting his Republican stablemates to issue awkward denunciations.

He stirred new controversy on Tuesday over comments interpreted by some as a threat of violence against rival Hillary Clinton.