White House Press Secretary Sean Spicer has apologised after declaring that Adolf Hitler did not use chemical weapons during World War Two.
"I mistakenly used an inappropriate and insensitive reference about the Holocaust and there is no comparison," he said. "For that I apologise. It was a mistake to do that."
Critics pointed out gas was used to kill Jews and others in the Holocaust.
Mr Spicer had been criticising Russia's support for the Syrian government.
The White House says Russia has been trying to deflect blame for a chemical weapons attack that killed 89 people.
US intelligence reports say the Syrian government used chemical weapons during air strikes. Syria denies this and Moscow has instead blamed rebel forces, which it says were storing chemical weapons which were hit in the raids.
The US responded by launching nearly 60 missiles on a Syrian military base.
Mr Spicer's comments, made during the Jewish festival of Passover, drew looks of astonishment from journalists.
Mr Spicer was asked to clarify his remarks during the news conference and garbled his words during the response.
"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 Asshad [sic] is doing."
He added: "There was not, in the, he brought them in to the Holocaust centres, I understand that, what I'm saying, in the way that Assad used them, where he went into towns dropped them down to innocent, into the middle of towns, it was brought to, so the use of it, and I appreciate the clarification, that was not the intent."
But the comments sparked backlash online, where Twitter users quickly condemned the remarks and noted that Hitler used gas to kill Jews, German citizens and political opponents of the Nazi Party.
Chelsea Clinton, the daughter of former President Bill Clinton and Hillary Clinton, was among those to rebuke Mr Spicer.
Some users also criticised Mr Spicer for referring to concentration camps as "Holocaust centres".
Senator Ben Cardin weighed in on Twitter, saying: "Someone get @PressSec a refresher history course on Hitler state #Icantbelievehereallysaidthat".
He added: "Sean Spicer: Really? The term you were looking for was concentration camps".
The Anne Frank Center called on President Donald Trump to sack Mr Spicer.
Executive director Steven Goldstein 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."
His statement added that the remarks were "the most evil slur upon a group of people" ever heard from a White House press secretary, and he should be fired.
In January, the White House was criticised for issuing a statement to commemorate International Holocaust Remembrance Day, which did not mention Jews, Judaism or anti-Semitism.
White House Chief of Staff Reince Priebus later defended the statement, arguing Mr Trump has family members who are Jewish and "there was no harm or ill will or offence intended by any of that".