White House Press Secretary Sean Spicer has sparked a furious backlash after saying Adolf Hitler "didn't even sink to using chemical weapons" during the Second World War.

In a daily briefing for reporters on Tuesday, Spicer criticised Russia's alliance with Syrian President Bashar al-Assad, who is accused of using chemical weapons against his citizens, by comparing the leader to Hitler.

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"We didn't use chemical weapons in World War Two," Spicer said. "You know, you had someone as despicable as Hitler who didn't even sink to using chemical weapons. So you have to, if you are Russia, ask yourself: Is this a country and a regime that you want to align yourself with?"

After a journalist pointed out that Hitler used gas chambers to kill millions of people during the Second World War, Spicer went on to talk about "Holocaust centers" - referring to Nazi concentration camps.

"When it comes to sarin gas, [Hitler] was not using the gas on his own people the same way that Assad is doing," he said.

"I understand your point. Thank you. I appreciate that. He brought them into the Holocaust centers, I understand that. I was saying in the way that Assad used them where he went into town, dropped them into the middle of town.

"I appreciate the clarification. That was not the intent."

After the briefing, Spicer again attempted to clarify his comments saying he was in no way "trying to lessen the horrendous nature of the Holocaust."

"I was trying to draw a distinction of the tactic of using airplanes to drop chemical weapons on population centers," he said. "Any attack on innocent people is reprehensible and inexcusable."

Spicer's latest gaffe also happened to take place on the first day of Passover, a Jewish celebration.

Sean Spicer
White House press secretary has sparked a massive debate after saying Adolf Hitler did not use chemical weapons. Alex Wong/Getty Images

He later apologised for his comments in an interview with CNN's Wolf Blitzer saying: "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. And for that I apologize. It was a mistake to do that."

Spicer's statements, however, were immediately widely mocked and triggered widespread outrage from Democrats, lawmakers, Holocaust scholars and social media users with many people calling for his firing.

"I hope @PressSec takes time to visit @HolocaustMuseum. It's a few blocks away," former first daughter Chelsea Clinton tweeted.

"Sean Spicer: 'That's a nice slow news day u have there. Would be a shame if someone were to...downplay the Holocaust & give props to Hitler," journalist Haley Byrd tweeted.

The Anne Frank Center also called on President Trump to fire Spicer and described him as a "national embarrassment" who engages in Holocaust denial.