Spicer later apologised and said he should not have made the comparison.
In Israel, the Yad Vashem memorial center urged Spicer to visit its website and expressed "deep concern regarding the inaccurate and insensitive use of terms related to the Holocaust" by the press secretary.
Hitler didn't use use chemical weapons on the battlefield but he and the Nazis used gas chambers to exterminate Jews.
Despite the apologies, Spicer's remarks provoked outrage around the globe from those who claimed he showed insensitivity to victims of the Holocaust. He also asked people to forgive him for his "mistake".
The global Jewish group, which has historical education trainings on the Holocaust for more than 1 million people, wrote an open letter to Spicer asking to come to the White House to hold an educational session about the topic.
"My goal now and then is to stay focused on Assad and I should have, and I'll make sure to stay in my lane when I talk about that", said the White House Press Secretary.
"I was obviously trying to make a point about the heinous acts that Assad had made against his own people last week, using chemical weapons and gas". He brought them into the Holocaust center; I understand that. Democratic leader Nancy Pelosi demanded Spicer be fired. Now, Spicer said they are just focusing on Assad. The White House generated criticism at the start of the year when a statement on worldwide Holocaust Remembrance Day did not make any reference to Jews.
"I clearly meant to say Concentration Clubs, ok?"
On Tuesday, Spicer said Syria's chemical weapons attack on civilians was worse than anything Hitler did.
"I know great people who would be exponentially better at this than Sean Spicer". The spokesperson suggested that even Adolf Hitler had not used the kind of chemical weapons that Assad had. "I remember all the disgusting pictures coming out of World War I from battlefield use of gas and the gas masks and what it did to people".
A reporter yelled out that the Jews had been gassed.
The White House Press secretary serves as a messenger between the president and the American people.