Sean Spicer just forgot the Holocaust happened.

White House Press Secretary Sean Spicer says Adolf Hitler “didn’t even sink to using chemical weapons” in a clumsy comparison to Syria that drew instant rebuke from Jewish groups and critics who noted it ignored Hitler’s use of gas chambers to exterminate Jews during the Holocaust.

Spicer was attempting to discuss the horror of the chemical weapons attack last week in Syria, which the Trump administration is blaming on President Bashar Assad.

“We didn’t use chemical weapons in World War II,” said Spicer, adding that “someone as despicable as Hitler … didn’t even sink to using chemical weapons.”

It was the second day in a row in which Spicer, President Donald Trump's main spokesman, appeared to struggle to articulate the president's foreign policy at a critical time. The White House generated criticism at the start of the year when a statement on international Holocaust Remembrance Day did not make any reference to Jews.

Asked by a reporter to clarify, Spicer delivered a garbled defence of his remarks in which he tried to differentiate between Hitler's actions and the gas attack on Syrian civilians last week. The attack in northern Syria left nearly 90 people dead, and Turkey's health minister said tests show sarin gas was used.

"I think when you come to sarin gas, there was no, he (Hitler) was not using the gas on his own people the same way that Assad is doing," Spicer said. "There was clearly ... I understand your point, thank you. There was not ... He brought them into the Holocaust centre I understand that."

"I appreciate the clarification. That was not the intent," he said.

The comparison to World War II appeared to be part of a message the administration was trying to deliver as it explains its tactics in Syria. Defense Secretary Jim Mattis noted in a separate briefing that "the intent was to stop the cycle of violence into an area that even in World War II chemical weapons were not used on battlefields".

Spicer, trying to clarify his Hitler answer—makes it worse: “He was not using the gas on his own people in the same way that Assad is doing” pic.twitter.com/SSA1P680z5

— Bradd Jaffy (@BraddJaffy) 11 April 2017

After the briefing, Spicer emailed a statement to reporters: "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 air planes to drop chemical weapons on population centres. Any attack on innocent people is reprehensible and inexcusable."

Democrats and Jewish organisations condemned the comments.

The New York-based Anne Frank Center for Mutual Respect called on Trump to fire Spicer, saying he denied that Hitler gassed Jews during the Holocaust.

Spicer's comments came on the first day of Passover and a day after the White House held a Seder dinner marking the emancipation of the Jewish people, a tradition started during the Obama administration.