Sean Spicer is fighting calls for his dismissal after he was forced to apologise for claiming during a press conference about Syria that even Adolf Hitler did not "sink" to using chemical weapons.

The White House press secretary was asked on Tuesday why he thought Russia would drop its longstanding alliance with Bashar al-Assad's government over last week's chemical attack.

He said: "We didn't use chemical weapons in World War Two, you know, you had a, someone as despicable as Hitler, who didn't even sink to the, to using chemical weapons.

Even Hitler didn't 'sink' to using chemical weapons like Assad has, Sean Spicer claims

"So you have to, if you're Russia, ask yourself is this a country that, and a regime, that you want to align yourself with."

The comments sparked immediate outrage, with the Anne Frank Centre for Mutual Respect saying Mr Spicer was the "most offensive press sec ever" on Twitter and calling for his resignation or dismissal.

The centre's executive director, Steven Goldstein, said: "On Passover no less, Sean Spicer has engaged in Holocaust denial, the most offensive form of fake news imaginable, by denying Hitler gassed millions of Jews to death.

"Spicer's statement is the most evil slur upon a group of people we have ever heard from a White House press secretary.

"Sean Spicer now lacks the integrity to serve as White House press secretary and President Trump must fire him at once."

Clarifying his remarks immediately after making them, Mr Spicer had said: "I think when you come to sarin gas there was no, he was not using gas on his own people the same way that Assad is doing. I mean, there's clearly ... He brought them into the Holocaust centres, I understand that.

"But I'm saying in the way that Assad used them where he went into towns, dropped them down to innocent—into the middle of towns, it was. The use of it—I appreciate the clarification, that was not the intent."

The embattled press secretary made his apology on CNN shortly afterwards.

He told the network: "I was obviously trying to make a point about the heinous acts that Assad had made against his own people last week, using chemical weapons and gas.

"Frankly, I mistakenly made an inappropriate and insensitive reference to the Holocaust, for which there is no comparison. For that I apologize. It was a mistake to do that."

House Democratic leader Nancy Pelosi of California said Mr Spicer was “downplaying the horror of the Holocaust” and should be fired.

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But Matt Brooks, executive director of the Republican Jewish Coalition, said that while “using the issue of the Holocaust or Hitler is problematic on many levels,” he believed Mr Spicer had “genuinely and sincerely apologised.”

“He's bent over backward to make clear those views are not his, not what he was trying to say,” Mr Brooks said, adding, “We accept that and move on.”

A senior member of Israel's government welcomed Mr Spicer's apology.

“Since he apologised and retracted his remarks, as far as (I) am concerned, the matter is over,” Intelligence and Transport Minister Israel Katz said in a statement, citing the “tremendous importance of historical truth and remembrance” of the victims of the Holocaust.