Donald Trump's press secretary has said he "let the president down", in an passionate apology for comments in which he suggested Adolf Hitler did not use chemical weapons and referred to concentration camps as "Holocaust centres".
Sean Spicer's efforts to redeem himself come amid reports of fierce infighting in the White House, with Mr Trump distancing himself from Steve Bannon, his chief strategist.
"I made a mistake. There's no other way to say it. I got into a topic that I shouldn't have and I screwed up," Mr Spicer said during an event at a museum in Washington.
"On both a personal level and a professional level that will definitely go down as not a very good day in my history."
Mr Spicer caused outrage when he suggested at a White House briefing that Bashar al-Assad had sunk lower than Hitler by using sarin gas in last week's attack in Syria. He claimed that Hitler "didn't descend to using chemical weapons".
It was the latest blooper from a White House riven with reports infighting and bitter rivalries between Mr Trump's top aides.
Mr Bannon is the main driver behind Mr Trump's nationalist agenda in the White House, and provided an ideological underpinning for Mr Trump's election campaign.
But in recent days he has clashed with Jared Kushner, Mr Trump's son-in-law, who controls a large policy portfolio and whose politics are reportedly more centrist.
Clearly exasperated by the fight, Mr Trump told the New York post: "Steve is a good guy, but I told them to straighten it out or I will." And he sought to undermine reports of Mr Bannon's influence, asserting that he is his "own strategist".
"I like Steve, but you have to remember he was not involved in my campaign until very late," he said. Mr Bannon, who has spoken admiringly of "Darth Vadar", the Star Wars character for the dark power that he wields, has long been a controversial figure.
In the White House, Mr Bannon is said to behave in an adversarial manner and to have isolated himself in the West Wing.
He however remains very popular with Trump voters, and removing him could risk alienating many among the president's base.