Leading Republican presidential candidate Donald Trump has found himself a new critic, and it's someone outside the United States.

The stepsister of famous World War II diarist Anne Frank heavily criticised Trump for his supposedly racist policies, even going as far as comparing the presidential aspirant to notorious German dictator Adolf Hitler.

In an essay published in Newsweek to mark the occasion of International Holocaust Remembrance Day, 86-year-old Eva Schloss, who currently resides in London, said the controversial business tycoon is "acting like another Hitler."

"If Donald Trump become(s) the next president of the U.S. it would be a complete disaster. I think he is acting like another Hitler by inciting racism," Schloss said, as quoted by CNN.

She particularly criticised Trump for his policies towards immigrants from the Middle East. The Republican presidential aspirant went as far as proposing the banning of Muslim immigrants in the U.S., saying some of them could be potential terrorists who could launch attacks on America.

The camp of the presidential hopeful did not give an immediate response to Schloss' comments.

Trump's newest critic became Frank's friend in Amsterdam after both their families left Nazi Germany in the 1930s. Frank and her mother died at the Bergen-Belsen concentration camp while Schloss survived Auschwitz.

The famous writer's father, Otto Frank, later married Schloss' mother, Fritzi, after World War II.

Schloss likewise aired her displeasure over the way the U.S. and Western European governments responded to the Syrian crisis. She said she can relate to the Syrian refugees saying it was similar "to what we went through" in Nazi-controlled Europe.

"I remember how upset the world was when the Berlin Wall was erected in 1961," Schloss said. "And now everybody is building walls again to keep people out. It's absurd."

"This is not just a European problem, it's a global problem. If countries as big as the U.S. and Canada would take in more people, then we would get much closer to a solution," Schloss commented on the refugee crisis.