Donald Trump's spokesperson has admitted he "screwed up" by suggesting Adolf Hitler did not use chemical weapons.
While discussing the nerve gas attack allegedly carried out by the Syrian regime, Mr Spicer told reporters that Hitler "didn't even sink to using chemical weapons".
Millions of Jews were killed in concentration camp gas chambers during the Holocaust.
Apologising for his comments, Mr Spicer said he had learned to "think before you speak".
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He said: "I made a mistake, there's no other way to say it. I got into a topic that I shouldn't have and I screwed up.
"I would ask for folks' forgiveness… I should have not tried to make a comparison.
"There's no comparing atrocities."
The White House press secretary added he had "let the President down" after what had been "an unbelievably successful couple of weeks" for Mr Trump.
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Mr Spicer faced calls to quit over his comments, which came a day after the start of Jewish holiday Passover.
Steven Goldstein, executive director of the Anne Frank Center for Mutual Respect, 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."
House Democratic leader Nancy Pelosi claimed Mr Spicer was "downplaying the horror of the Holocaust", while senator Ben Cardin called for "someone (to) get the press secretary a refresher history course on Hitler".
It is not the first time Mr Spicer has been caught up in controversy during his time in the White House.
In January, the spokesman was ridiculed after claiming Mr Trump had "the largest audience ever" for his inauguration - despite images showing large, empty spaces around the Washington Monument.
A day later he defended his attacks against the media , telling reporters: "I think sometimes you can disagree with facts."
Last month, he was mocked on social media after being pictured wearing his American flag pin upside down .