Republican presidential candidate Donald Trump talks with members of his staff while visiting the Chez Vachon restaurant. Photo: AP/PTI

Germany's Vice-Chancellor Sigmar Gabriel said t'oday that US billionaire Donald Trump, frontrunner for the Republican presidential nomination, poses a threat to peace, social cohesion and prosperity.

Gabriel labelled the brash real estate mogul a "right-wing populist" who, like Marine Le Pen of France and Geert Wilders of the Netherlands, promises voters unsettled by globalisation a return to a protected "fairytale world".

"Whether Donald Trump, Marine Le Pen or Geert Wilders -- all these right-wing populists are not only a threat to peace and social cohesion but also to economic development," the centre-left Social Democrat told the Welt am Sonntag newspaper.

"The right-wing populists promise their followers a return to a fairytale world where economic life happens only within the borders of their own nation," said Gabriel, vice-chancellor to conservative Angela Merkel.

Gabriel, who is also economy minister, stressed that countries don't prosper in self-imposed isolation and that, in export powers like Germany and elsewhere, "we need to make the effort to explain how to shape globalisation in a fair way".

Merkel was asked in a separate interview, by Bild am Sonntag newspaper, to comment on Trump, and replied only that "I don't know him personally".

Questioned about his harsh attacks on her liberal refugee policy, which he has called "insane", Merkel said that "I see no reason to reply to that".

Merkel did however praise Democrat frontrunner Hillary Clinton, saying that "I appreciate her deep political experience, her commitment to women's rights, to family and health care".

"I appreciate her strategic thinking and that she is a strong supporter of the transatlantic partnership," Merkel added about the former US secretary of state. "Whenever I have worked with Hillary Clinton, it was a great joy.