White House press secretary Sean Spicer caused an uproar on Tuesday when he said that Nazi leader Adolf Hitler, whose regime gassed millions of Jews during the Holocaust, did not use chemical weapons during World War II.

Spicer stumbled while trying to make a point about Syrian President Bashar Assad, who is accused of employing chemical weapons against his citizens. Russia's government has backed Assad.

"We did not use chemical weapons in World War II. You had someone as despicable as Hitler who did not even sink to using chemical weapons," Spicer said. "If you are Russia, ask yourself, is this a country and regime that you want to align yourself with?"

Given an opportunity to clarify his comments, Spicer misspoke again by trying once more to draw a distinction between Assad and Hitler, whom the press secretary said did not gas "his own people."

"When it comes to sarin gas, [Hitler] was not using the gas on his own people the same way that Assad is doing," Spicer said to audible groans from some reporters. He also referred to concentration camps as "Holocaust centers."

"I understand your point. Thank you. I appreciate that. He brought them into the Holocaust centers, I understand that. I was saying in the way that Assad used them where he went into town, dropped them into the middle of town. I appreciate the clarification. That was not the intent."

After the briefing, Spicer clarified his comments again on Twitter.

"In no way was I trying to lessen the horrendous nature of the Holocaust," the spokesman said. "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."

The remarks were widely mocked by Democrats and criticized by groups such as the Anne Frank Center for Mutual Respect, which called for Spicer to 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," said Steven Goldstein, executive director of the Anne Frank Center.

Spicer's remarks drew rebukes from numerous lawmakers, including Sen. Brian Schatz (D-Hawaii), who is Jewish.

Spicer's remarks are likely to raise speculation about his job future. The press secretary has repeatedly stumbled at the podium, most memorably the day after President Trump's inauguration, when he falsely claimed that more people attended it than any previous inauguration

The comments about Hitler were particularly damaging given criticisms that the White House has not done enough to fight anti-Semitism.

The White House failed to mention Jews in a late January statement marking Holocaust Remembrance Day. During a February news conference, Trump told a Jewish reporter to sit down after he asked about threats against Jewish Community Centers.

The president made his first public condemnation of the incidents five days later.

The gaffe prevented Spicer from truly augmenting the White House's message on Syria, even as Secretary of State Rex Tillerson landed in Moscow for high-level meetings.

Spicer was trying to pressure Russia to abandon Assad, whom U.S. officials have accused of launching the sarin gas attack earlier this month that provoked Trump to retaliate with a missile strike on a Syrian airfield.

Updated at 3:17 p.m.