U.S. human rights watchdog Anne Frank Center for Mutual Respect urged President Donald Trump to fire White House Press Secretary Sean Spicer over his comments Tuesday comparing Adolf Hitler’s murderous Nazi tactics of the Holocaust to those of the deadly chemical attack launched in Syria last week by President Bashar al-Assad.

"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," Steven Goldstein, executive director of the human rights watchdog, said in the statement published on Facebook. He also said that by making the remark during the popular Jewish public holiday of Passover, Spicer has "engaged in Holocaust denial, the most offensive form of fake news imaginable, by denying Hitler gassed millions of Jews to death."

Read: A Timeline Of Controversy From Trump’s Press Secretary

In attempting to decry Russia's link to Syria and trying to allege that Moscow knew about the attack, Spicer said in a press conference Tuesday: “You know, you had someone as despicable as Hitler who didn’t even sink to using chemical weapons.”

His comments faced immediate backlash and were criticized on social media. "Even Hitler," "Sean Spicer" and "Holocaust Centers" began trending on Twitter Tuesday almost immediately.

Following the trends, White House reporters asked Spicer if he wanted to clarify what he meant, to which he replied: “He was not using the gas on his own people in the same way that Assad is doing.”

After the presser ended, Spicer clarified his comments through White House correspondents: "In no way was I trying to lessen the horrendous nature of the Holocaust, however, I was trying to draw a contrast of the tactic of using airplanes to drop chemical weapons on innocent people."

Spicer later released a statement late Tuesday afternoon apologizing for his comments.

"I mistakenly used an inappropriate, insensitive reference to the Holocaust for which there is no comparison," Spicer told CNN.

House Democratic leader Nancy Pelosi also called for Spicer's removal from his position in the White House as press secretary.

"While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust," she said in a statement. "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."