Israel's foreign minister has denounced as "unforgivable" remarks by his Russian counterpart that Nazi leader Adolf Hitler "had Jewish blood".
Russian FM Sergei Lavrov made the comments to try to justify Russia's portrayal of Ukraine as "Nazi" despite the fact that its president is Jewish.
Israel's foreign ministry summoned Russia's ambassador for "clarification" and demanded an apology.
Nazi Germany murdered six million Jews in the Holocaust in World War Two.
Mr Lavrov made the remarks in an interview on Italian TV programme Zona Bianca on Sunday, days after Israel marked Holocaust Remembrance Day, one of the most solemn occasions in the Israeli calendar.
When asked how Russia can claim that it is fighting to "de-Nazify" Ukraine when President Volodymyr Zelensky is himself Jewish, Mr Lavrov said: "So what if Zelensky is Jewish."
"The fact does not negate the Nazi elements in Ukraine. I believe that Hitler also had Jewish blood," adding that "some of the worst anti-Semites are Jews."
Israel's foreign minister reacted furiously to Mr Lavrov's statement, accusing him of anti-Semitism.
Mr Lavrov was also condemned by the head of Israel's Yad Vashem Holocaust memorial, Dani Dayan.
"Most of his remarks are absurd, delusional, dangerous and deserving of any condemnation," he tweeted. "Lavrov deals with the reversal of the Holocaust: turning the victims into criminals, based on the promotion of a completely unfounded claim that Hitler was of Jewish descent."
There have for decades been unproven claims that Hitler's unidentified paternal grandfather was Jewish, fuelled by an assertion by Hitler's lawyer Hans Frank.
In his memoir, published in 1953, Frank said he had been instructed by Hitler to investigate rumours that he had Jewish ancestry. Frank said he uncovered evidence that Hitler's grandfather was indeed Jewish - though the claim, which has gained ground among conspiracy theorists, has been dismissed by mainstream historians.
Russia has repeatedly said that one of its aims in the war against Ukraine is what it calls the "de-Nazification" of the country, making the unfounded claim that its government is steeped in Nazi ideology.
Israel, which has close relations with both Ukraine and Russia, has been at the forefront of international mediation efforts to try to stop the fighting.