White House Press Secretary Sean Spicer has said his gaffe about Adolf Hitler not using chemical weapons was "inexcusable and reprehensible".
He said "there's no comparing atrocities" after Tuesday's remark seeking to draw a distinction between Syria's leader and the German dictator.
"I got into a topic that I shouldn't have and I screwed up," he said at a media forum in Washington.
"I would ask obviously for folks' forgiveness," he added.
He was accused of being insensitive to Jews and Holocaust survivors.
He told reporters at a White House news conference on Monday that Hitler "was not using gas on his own people the same way that Asshad [sic] is doing".
Mr Spicer had been criticising Russia's support for Syrian President Bashar al-Assad, who is accused of launching a chemical weapons attack earlier this month that left 89 people dead.
On Wednesday morning, the press secretary said his comments had distracted from President Donald Trump's recent actions, which included responding to the suspected deadly gas attack with a cruise missile strike and a bilateral with Chinese President Xi Jinping.
"On a professional level it's disappointing because I think I've let the president down," he said at a Newseum forum on Mr Trump's first 100 days.
But his comments, which came during the Jewish festival of Passover, sparked online backlash and calls to remove him from his White House post.
The Anne Frank Center and Democratic Leader Nancy Pelosi both called on Mr Trump to fire him while some criticised Mr Spicer for referring to concentration camps as "Holocaust centres".
"Either he is speaking for the President, or the President should have known better than to hire him," she said in a statement.