A senior US official has said Washington is "dismayed" by Turkish prime minister Recep Tayyip Erdogan's description of Zionism as "a crime against humanity."
The official, speaking anonymously, added that Erdogan's comments are "particularly offensive" and will have a "corrosive effect" on US-Turkish relations.
The comments come as US Secretary of John Kerry flies to Ankara for a meeting with Erdogan.
"I am sure the secretary will be very clear about how dismayed we were to hear it," the official added.
Speaking at the Alliance of Civilizations' forum in Vienna, Erdogan urged UN secretary-general Ban Ki-moon to recognise Islamophobia as a crime against humanity "just like Zionism, anti-Semitism and fascism".
Israeli prime minister Benjamin Netanyahu reacted with fury, saying in a statement: "I had thought that such dark and libelous comments were a thing of the past."
Israeli foreign ministry spokesman Yigal Palmor also criticised Erdogan, saying: "Zionism is the national movement of the Jewish people and to deny any people their right to self-determination and to their national movement is absurd. We will not dignify such nonsense with any future comment," he said, claiming that Erdogan's remarks were "hollow words that only reflect ignorance".
The remarks were immediately slammed as "Ahmadinejad-style pronouncements" by the Geneva-based UN watch, a non-governmental organisation that monitors the performance of the UN "by the yardstick of its own charter".
UN Watch criticised Ki-moon, who was present on the stage, because he stayed silent and did not speak out condemning the speech.
Ki-moon later released a statement saying Erdogan's description of Zionism was "hurtful and divisive".
"The secretary-general believes is it is unfortunate that such hurtful and divisive comments were uttered at a meeting being held under the theme of responsible leadership," he said.