The US press secretary says it was "inappropriate and insensitive"
Donald Trump's spokesman has issued an apology and admitted he made a mistake by suggesting Adolf Hitler did not use chemical weapons.
Sean Spicer faced calls to be sacked after comparing Bashar al Assad to the Nazi leader during his daily briefing.
He was discussing the chemical attack allegedly carried out by the Syrian regime.
Mr Spicer told Fox News: "I was trying to describe the attack on his own people - the use of chemical weapons.
"Frankly, I mistakenly made an inappropriate and insensitive reference to the Holocaust for which, frankly, there's no comparison.
"Obviously, that's not what I wanted to do and especially during this week, I regret using that term.
"I apologise and hope that we continue to focus on the President's decisive action that he took to deal with the situation in Syria."
When comparing Mr Assad's actions to those of the Nazi leader, he told reporters during a news conference that Hitler "didn't even sink to using chemical weapons".
Millions of people were killed in concentration camp gas chambers during the Holocaust.
The backlash had begun on social media even before Mr Spicer had completed the briefing.
Steven Goldstein, executive director of the Anne Frank Center for Mutual Respect, said: "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 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."
The Democrats also called for Mr Spicer to be sacked, with party leader in the House Nancy Pelosi saying in a statement: "While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust.
"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."
Mr Spicer had been answering questions about last week's attack in northern Syria which left nearly 90 people dead after sarin gas was allegedly used.
The US has blamed Mr Assad and said he acted with Russia's support.
Mr Trump's White House faced accusations of anti-Semitism just days after he took office when a statement marking International Holocaust Remembrance Day neglected to mention Jewish people.
It later said the decision was taken because other victims also suffered and died. Hillary Clinton's running mate, Tim Kaine, called that "Holocaust denial".
Mr Spicer has frequently run into controversy and is often lampooned on US comedy sketch shows.