Yair Netanyahu, son of the Israeli prime minister, said leftists are more dangerous than neo-Nazis
Israel's president, Reuven Rivlin, expressed shock on Wednesday over anti-Semitism at a recent US white supremacist rally, but said American leaders would succeed in dealing with "this difficult challenge".
Meanwhile, world leaders, including Britain's prime minister Theresa May, criticised Donald Trump for blaming "both sides" on Tuesday for the unrest in Charlottesville, Virginia, over the weekend.
White supremacists clashed with counter-protesters and a car ploughed into the rally opponents, killing one woman and injuring 19 other people. The driver, 20-year-old Ohio man James Fields, said to harbour Nazi sympathies, was charged with murder.
Rivlin wrote to the Conference of Presidents of Major American Jewish Organizations to convey his "support and solidarity for the American Jewish community" after the unrest in Charlottesville.
"The very idea that in our time we would see a Nazi flag - perhaps the most vicious symbol of anti-Semitism - paraded in the streets of the world's greatest democracy, and Israel's most cherished and greatest ally, is almost beyond belief," he said.
Rivlin's remarks came a day after Israel's prime minister, Benjamin Netanyahu, condemned "neo-Nazism and racism" in the Virginia college town.
Trump's comments on Tuesday were praised by white supremacists and neo-Nazis, including former KKK leader David Duke.
"This man is doing absolutely everything in his power to back us up and we need to have his back," Andrew Anglin wrote on the DailyStormer, a neo-Nazi, anti-Semitic website that acts as a hub of the extreme right.
However, Yair Netanyahu, the Israeli prime minister's son, said leftist groups pose a greater threat to Israel than neo-Nazis, whom he described as a dying "breed".
He called Black Lives Matter activists thugs, saying that they hate Israel and are getting stronger.
The UK prime minister, widely criticised at home for cultivating close ties to Trump during his first half year in office, spoke out against the US president.
"There's no equivalence, I see no equivalence between those who propound fascist views and those who oppose them and I think it is important for all those in positions of responsibility to condemn far-right views wherever we hear them," May said.
In Europe, even far-right parties that have previously welcomed Trump's nationalist message were critical of his stance.
"These were white supremacists and racists. They need to be condemned in very clear terms," said Florian Philippot, vice president of France's National Front and the manager of Marine Le Pen's campaign for the French presidency.