Donald Trump's press secretary Sean Spicer is facing an international backlash and calls to resign or be fired afterclaiming that Adolf Hitler did not use chemical weapons during World War Two.

During his daily press briefing yesterday, Spicer compared the Syrian President Bashar Assad to Hitler while responding to a question about an alleged chemical weapons attack in Syria last week.

'You had a — someone as despicable as Hitler, who didn't even sink to using chemical weapons,' Spicer said.

Later in the briefing, Spicer claimed that Hitler 'was not using the gas on his own people the same way that Assad is doing'.

The Anne Frank Centre for Mutual Respect led the criticism, calling for the President to sack Spicer 'at once'.

Steven Goldstein, the executive director of the centre, said in a statement: '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.'

He called Spicer's comments 'the most evil slur upon a group of people we have ever heard from a White House press secretary,' adding: 'Sean Spicer now lacks the integrity to serve as White House press secretary, and President Trump must fire him at once.'

Spicer later apologised for the comments, telling Fox News: 'I was trying to describe the attack on his own people - the use of chemical weapons. Frankly, I mistakenly made an inappropriate and insensitive reference to the Holocaust for which, frankly, there's no comparison.

'Obviously, that's not what I wanted to do and especially during this week, I regret using that term.

'I apologise and hope that we continue to focus on the President's decisive action that he took to deal with the situation in Syria.'

Democratic House leader Nancy Pelosi also called on Spicer to be fired, saying: 'While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust. Sean Spicer must be fired, and the President must immediately disavow his spokesman's statements. Either he is speaking for the President, or the President should have known better than to hire him.'

In Israel, Transportation Minister Israel Katz (of the Likud party) called Spicer's statement 'grave' and 'outrageous,' adding that there is a 'moral obligation that precedes diplomatic policy'.

The Knesset Israel-US Relations Caucus Chairman Nachman Shai (Zionist Union) said Spicer's statement was ignorant and outrageous and called Trump to renounce them.

'The White House urgently needs a history teacher,' said Shai. 'Ignorance is not acceptable...Hitler used chemical weapon of mass destruction in a large scale that human history hadn't seen until then, and since then.'

Referring to the White House statement commemorating International Holocaust Remembrance Day in which Jews were not mentioned as victims, Shai added: 'This a second failure of the White House recently, regarding the biggest tragedy that occurred to the Jewish people and took place in the twentieth century...Where are the president's tweets when you need them?'