Spicer's apology came on national television yesterday for comparing the Nazi leader with Syrian President Bashar al-Assad, who has been accused of using nerve agent against Syrians.
"I mistakenly used an inappropriate and insensitive reference to the Holocaust for which, frankly, there is no comparison," Spicer told CNN hours after he made the remarks.
"I was obviously trying to make a point about the heinous acts that Assad made against his own people last week using chemical weapons and gas," he said in an effort to control the damage done by his remarks, which drew strong reactions from political leaders, the Jew community and human rights bodies.
House Democratic Whip Steny H Hoyer said Spice's remarks were "indicative of the extraordinary lack of knowledge, perspective, and sensitivity that the Trump administration has brought to Washington."
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Congresswoman Jan Schakowsky said: "It is clear that this administration needs a history lesson and a staff shake-up."
Nita M Lowey, ranking member of the House Appropriations Committee, said it was disturbing that US President Donald Trump's chief spokesman has "either a cavalier aversion to facts, is completely unmoored from reality, or both."
"So you have to, if you're Russia, ask yourself is this a country that you and a regime that you want to align yourself with?"
Later, in a statement to White House reporters, Spicer said in no way he was trying to lessen the horrendous nature of the Holocaust. "Any attack on innocent people is reprehensible and inexcusable," he said.