New York: White House press secretary Sean Spicer, besieged by widespread condemnation and calls for his resignation, made a televised apology on Tuesday afternoon for a baffling series of gaffes in which he favourably compared Adolf Hitler to President Bashar al-Assad, and claimed the Nazi leader had not used chemical weapons against his own people.

Democrats and the Anne Frank Centre for Mutual Respect were among those calling for the senior official, already a frequent target of criticism and ridicule for his combative and at times bumbling style, to be dumped from the role.

In making his unreserved apology, Spicer made a further couple of gaffes, mispronouncing the name of the Syrian leader and stating that he didn't want to distract from President Donald Trump's attempt to "destabilise the region".

Spicer's unforced errors began during a high stakes press conference on Tuesday, in which he was trying to explain the White House's position on several critical international policy fronts, including the US strikes on Syria, mounting tension with Russia and Trump's sabre-rattling against North Korea on Twitter that morning.

During an answer on the recent chemical attack in Syria, Spicer framed the depravity of the act using a comparison with Nazi Germany, saying: "You had someone as despicable as Hitler, who didn't even sink to chemical weapons."

Hitler's regime slaughtered millions of Jews and many others during the Holocaust; many were murdered using the poisonous gas "Zyklon B", or hydrogen cyanide, in gas chambers.

Asked to clarify his remark by shocked journalists, Spicer gave a rambling answer citing "Holocaust centres" and mispronouncing the name of the Syrian leader.

"I think when you come to sarin gas, there was no - he was not using the gas on his own people the same way that Ashad is doing ... he brought them into the Holocaust centres and 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 ??? the use of it. I appreciate the clarification there, that was not the intent."

As the responses drew a fierce backlash on social media, Spicer, who repeatedly mangled the name of the Australian Prime Minister during several press conferences earlier this year, then put out two separate statements trying to clarify what he meant by comparing Assad and Hitler.

"In no way was I 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 centres. Any attack on innocent people is reprehensible and inexcusable."

But within hours, the series of comments sparked confusion, ridicule as well as multiple calls for the press secretary's resignation.

Steven Goldstein, executive director of the Anne Frank Centre for Mutual Respect, went as far as to accuse the press secretary of engaging in Holocaust denial.

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

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

House Minority Leader Nancy Pelosi said Spicer must be fired and that the President should disavow his comments.

"While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust."

Others, without defending the remarks, rejected the suggestion Spicer was denying the Holocaust.

"Spicer is historically illiterate, but he didn't deny the Holocaust, as his doltish reference to 'Holocaust Centres' makes clear," author Michael Weiss wrote on Twitter.

Spicer later appeared on CNN trying to defuse the anger and fallout over the comments, offering a full apology and withdrawing the comparison between Hitler and Assad.

"I mistakenly used an inappropriate and insensitive reference to the Holocaust for which, frankly, there is no comparison. For that, I apologise. It was a mistake to do that," he said.

"I should have stayed focused on the Assad regime and the dangers they have brought to their own people, and the terrible atrocities they did, and to drag any other comparison into this was not appropriate."

Pressed on whether Trump had pushed him to make the apology, Spicer said he was doing it because he realised he had made a mistake, and "did not want to be a distraction from the President's agenda".

But during that interview with Wolf Blitzer, he again mispronounced the name of the Syrian leader, saying "Bashar al-Asee ... Bashar al-Assad."

Blitzer made a curt interjection: "Bashar al-Assad. I know you've mispronounced his name a few times."

During the interview, Spicer also said he didn't want to distract from Trump's attempt to "destabilise the region", when he presumably meant stabilise.