He then said: "I think when it comes to sarin gas he (Hitler) was not using the gas on his own people the same way that Assad is doing.

"There was clearly not...he (Hitler) brought them into the Holocaust centre and I understand that. What I'm saying is Assad used them where he went into towns and dropped them down into the middle of towns...the use of it, I appreciate the clarification, that was not the intent."

The comment came during the Jewish holiday of Passover.

Following his briefing Mr Spicer later sought to further clarify his comments.

In a statement he said: "In no way was I trying to lessen the horrendous nature of the Holocaust.

"However, I was trying to draw a contrast of the tactic of using airplanes to drop chemical weapons on innocent people.

"Any attack on innocent people is reprehensible and inexcusable."

The US-based Anne Frank center for Mutual Respect called on Mr Trump to fire Mr Spicer and he was widely criticised on social media.