WASHINGTON • Calls for White House press secretary Sean Spicer's resignation are mounting in a furious backlash after he suggested that President Bashar al-Assad of Syria was guilty of acts worse than Adolf Hitler and asserted that Hitler had not used chemical weapons.

In criticising Mr Assad's alleged use of chemical weapons during a Tuesday briefing, Mr Spicer said that even Hitler did not sink to that level of warfare and "was not using the gas on his own people in the same way that Assad is doing", despite Hitler's use of gas chambers to kill millions of Jews and others, reported the Washington Post.

Following hours of controversy, Mr Spicer walked back his remarks late in the day, but it appeared to be too late. Among the chorus of voices calling for Mr Spicer to be fired was Anne Frank Centre for Mutual Respect executive director Steven Goldstein, a frequent critic of President Donald Trump, who released a statement accusing Mr Spicer of having "engaged in Holocaust denial", the "most offensive form of fake news possible".

House Minority Leader Nancy Pelosi, a California Democrat, said Mr Trump should fire Mr Spicer, accusing him of "downplaying the horror of the Holocaust".

"Someone get @PressSec a refresher history course on Hitler stat #Icantbelievehereallysaidthat," Maryland Democratic Senator Ben Cardin, who is Jewish, wrote on Twitter, reported the Daily Mail.

German Chancellor Angela Merkel's spokesman yesterday said contemporary comparisons with Nazi atrocities were generally ill-advised. "Any comparison of current situations with the crimes of National Socialism leads to nothing good," the spokesman, Mr Steffen Seibert, told reporters when asked about Mr Spicer's remarks, reported Agence France-Presse.

Nevertheless, a senior member of Israel's government welcomed Mr Spicer's apology yesterday. "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, reported Reuters.

Mr Spicer brought up Hitler unprompted during Tuesday's White House briefing when asked about the alliance between Mr Assad and Russia.

"We didn't use chemical weapons in World War II," Mr Spicer said, reported The New York Times. "You know, you had someone as despicable as Hitler who didn't even sink to using chemical weapons."

Asked to clarify his remarks, Mr Spicer then acknowledged that Hitler had used chemical agents but maintained that there was a difference. "I think when you come to sarin gas, he was not using the gas on his own people the same way that Assad is doing," Mr Spicer said, incorrectly, before mentioning "Holocaust centres", an apparent reference to Nazi death camps.

Reporters tried to correct Mr Spicer, with one person shouting out: "He gassed the Jews!"

Before the briefing was even over, White House press aides realised the magnitude of Mr Spicer's mistake. Shortly after he stepped away from the lectern, Mr Spicer put out a statement again trying to explain what he meant, reported The Washington Post.

He later made an apology on live TV, but criticism continued on social media and elsewhere.