White House press secretary Sean Spicer on Tuesday attempted to argue that Syrian President Bashar Assad was even worse than Adolf Hitler, because the German dictator “didn’t even sink to using chemical weapons.” The Nazis gassed millions of Jews in concentration camps.

While briefing reporters on whether U.S. relations with Syrian ally Russia will change in the wake of the Syrian regime unleashing chemical weapons last week, Spicer tried to compare Assad to Hitler.

“We didn’t use chemical weapons in World War II. You know, you had someone as despicable as Hitler who didn’t even sink to using chemical weapons,” Spicer said. “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?’”

When a reporter asked him to clarify his remarks, Spicer responded: “He was not using gas on his own people in the same way that Assad is doing,” adding that Hitler “brought them into the Holocaust centers,” apparently referring to the gas chambers.

MSNBC quickly fact-checked Spicer in a simple but effective chyron.

MSNBC with the winning chyron. pic.twitter.com/BxVFpBooVF — Jennifer Bendery (@jbendery) April 11, 2017

The U.S. Holocaust Museum also seemingly fact-checked Spicer in a tweet.

WATCH: Footage from our collection shows what US forces discovered when they liberated #Buchenwald. pic.twitter.com/jySQOWM6Lf — US Holocaust Museum (@HolocaustMuseum) April 11, 2017

Spicer later tried to walk back his remarks again in a statement.

“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 centers. Any attack on innocent people is reprehensible and inexcusable,” he said.

Democratic Party Chairman Tom Perez called on the White House to apologize, calling Spicer’s remarks “shameful.”

The @WhiteHouse absolutely must apologize for their callous and inaccurate statement about the Holocaust. Shameful. — Tom Perez (@TomPerez) April 11, 2017

The Anne Frank Center released a scathing statement saying Spicer should be fired.

“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 most evil slur upon a group of people we have ever heard from a White House press secretary,” the center’s executive director, Steven Goldstein, said. “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 (D-Calif.) also called for Spicer’s firing in a statement later Tuesday.

“While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust,” she said. “Sean Spicer must be fired, and the President must immediately disavow his spokesman’s statements. Either he is speaking for the President, or the President should have known better than to hire him.”

Spicer later apologized for the comments in an interview with Wolf Blitzer on CNN. “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, and frankly I mistakenly used an inappropriate and insensitive reference to the Holocaust, for which frankly there is no comparison. And for that I apologize, it was a mistake to do that,” he said.

This story has been updated to include Spicer’s apology.