"I made a mistake, there is no other way to say it", Mr Spicer told a forum at the Newseum in Washington, adding that the damage was especially unfortunate because "it's a very holy week for the Christian people and the Jewish people". "There was clearly. I understand your point, thank you". "He brought them into the Holocaust center, I understand that".
"In no way was I trying to lessen the horrendous nature of the Holocaust", Spicer said.
The attack in northern Syria left almost 90 people dead, and Turkey's health minister said tests show sarin gas was used.
U.S. defence secretary Jim Mattis noted in a separate briefing that "the intent was to stop the cycle of violence into an area that even in World War II chemical weapons were not used on battlefields".
When asked whom he was directing his apology to, Spicer said, "anyone offended by the comments". A spokesman for German Chancellor Angela Merkel said comparing Nazi war crimes to present-day situations "leads to nothing good".
Democrats and Jewish organisations condemned Mr Spicer's comments.
"I love it", Spicer said of his job.
"Spicer's statement is the most evil slur upon a group of people we have ever heard from a White House press secretary".
But Matt Brooks, executive director of the Republican Jewish Coalition, said that while "using the issue of the Holocaust or Hitler is problematic on many levels", he believed Spicer had "genuinely and sincerely apologized".
House Democratic leader Nancy Pelosi claimed Mr Spicer was "downplaying the horror of the Holocaust", while senator Ben Cardin called for "someone (to) get the press secretary a refresher history course on Hitler".
The White House generated criticism at the start of the year when a statement on global Holocaust Remembrance Day did not make any reference to Jews.
He noted that Trump had bombed a Syrian air base in a forceful response to chemical weapons use by Assad's government and said Trump's hosting of Chinese President Xi Jinping last week had produced "tremendous progress". He had said: "We didn't use chemical weapons in World War II".