The White House has sparked outrage after Sean Spicer claimed that not even Hitler sank to using chemical weapons in the way it is believed Bashar al-Assad's government did in Syria last week.
The White House press secretary was asked by a reporter why Russia would pull out of its alliance with Mr Assad now, given its longstanding nature.
Mr Spicer said during a briefing on Tuesday: "We didn't use chemical weapons in World War Two, you know, you had a, someone as despicable as Hitler, who didn't even sink to the, to using chemical weapons.
"So you have to, if you're Russia, ask yourself is this a country that, and a regime, that you want to align yourself with.
"You have previously signed on international agreements rightfully acknowledging that the use of chemical weapons should be out of bounds by every country. Russia put their name on the line.
"So it's not a question of how long that alliance has lasted, but at what point do they recognise that they are now getting on the wrong side of history in a really bad way."
He was speaking after the US and UK called on Russia to drop its support for the Assad regime, following last week's chemical attack on the village of Khan Sheikhoun in Idlib province.
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) April 11, 2017
Foreign Secretary Boris Johnson said on Monday: "We call on Russia to do everything possible to bring about a political settlement in Syria and work with the rest of the international community to ensure that the shocking events of the last week are never repeated.
"We deplore Russia's continued defence of the Assad regime even after the chemical weapons attack on innocent civilians."
Asked to clarify his remarks, Mr Spicer said: "I think when you come to sarin gas there was no, he was not using gas on his own people the same way that Assad is doing. I mean, there's clearly ... He brought them into the Holocaust centres, I understand that.
"But 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. The use of it—I appreciate the clarification, that was not the intent."
A caption on the live video stream of Mr Spicer's briefing, carried by MSNBC, read: "White House: Hitler didn't 'sink to level of using chemical weapons' like Syrian leader (Hitler gassed millions)".