"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", said Spicer to Wolf Blitzer.
After the briefing, Spicer emailed reporters a statement that said: "In no way was I trying to lessen the horrendous nature of the Holocaust".
He said his original comments were "inexcusable and reprehensible". "We had someone as despicable as Hitler who didn't even sink to using chemical weapons". A spokesman for German Chancellor Angela Merkel said comparing Nazi war crimes to present-day situations "leads to nothing good".
The US Holocaust Museum did not mention Spicer's comments directly, but sent out a tweet shortly after the briefing that showed graphic footage of dead bodies US forces found while liberating the Buchenwald concentration camp.
"I kinda can't wait every day to see what this guy says", Stern told his sidekick Robin Quivers during his Tuesday broadcast. Ben Cardin, D-Md., said on Twitter, "Someone get @PressSec a refresher history course on Hitler stat (hashtag)#Icantbelievehereallysaidthat".
Ms Pelosi, the Democrats' leader in the House of Representatives, said that Mr Spicer was "downplaying the horror of the Holocaust" and should be sacked, as did the New York-based Anne Frank Centre for Mutual Respect.
Two conservative news outlets had made a similar comparison between Hitler and Assad earlier in the week.
There are historical facts proving Hitler did use gas chambers to kill six million Jewish people during the Holocaust. Earlier in the year, the White House generated criticism by issuing a statement on worldwide Holocaust Remembrance Day that did not mention Jews.
White House Press Secretary Sean Spicer speaks during "The President and the Press: The First Amendment in the First 100 Days" forum at the Newseum in Washington, DC, April 12, 2017.
But Hennen pressed him for more, saying of Spicer, "I think what he was saying is that Hitler didn't take chemical weapons out in a battle-like form and do what Assad did with these children".