White House Press Secretary Sean Spicer has apologised for making an "inappropriate and insensitive" comparison to the Holocaust in earlier comments about Syrian President Bashar al-Assad's use of chemical weapons - remarks that drew instant rebuke from Jewish groups and critics.

"In no way was I trying to lessen the horrendous nature of the Holocaust," Spicer said in a statement on Tuesday.

"I was trying to draw a distinction of the tactic of using airplanes to drop chemical weapons on population centres. Any attack on innocent people is reprehensible and inexcusable."

During the daily White House briefing, Spicer told reporters that Adolf Hitler "didn't even sink to using chemical weapons". Critics noted the remark ignored Hitler's use of gas chambers to exterminate Jews during the Holocaust.

The White House generated criticism at the start of the year when a statement on international Holocaust Remembrance Day did not make any reference to Jews.

Condemnation

The gas attack on the rebel-held town of Khan Sheikhoun last week left dozens of people dead, and Turkey's health minister said tests show sarin gas was used.

"You had someone as despicable as Hitler who didn't even sink to using chemical weapons," Spicer said when asked about Russia's alliance with the Syrian government.

The Nazis murdered six million Jews during World War II. Many Jews as well as others were killed in gas chambers in European concentration camps.

When a reporter asked Spicer if he wanted to clarify his comments, he said Hitler had not used chemical weapons in a battlefield situation the same way the Syrian president did.

"I think when you come to sarin gas, there was no, he was not using the gas on his own people the same way that Assad is doing."

He said Hitler "brought them into the Holocaust centre," while Assad "went into towns, dropped them down into innocent - into the middle of towns".

Democrats and Jewish organisations condemned the comments.

House Democratic leader Nancy Pelosi said in a statement that Spicer was "downplaying the horror of the Holocaust" and should be fired.

Senator Ben Cardin said on Twitter, "Someone get @PressSec a refresher history course on Hitler stat (hashtag)#Icantbelievehereallysaidthat."

The New York-based Anne Frank Centre for Mutual Respect called on Trump to fire Spicer, saying he denied that Hitler gassed Jews during the Holocaust.

Jeremy Ben-Ami, president of liberal pro-Israel advocacy group J Street, called the comments "unforgivable".

Quick note to @PressSec: By "holocaust centers," you must mean Concentration Camps - where my relatives died - many of them gassed. — Jeremy Ben-Ami (@JeremyBenAmi) April 11, 2017

Lee Zeldin, a Jewish Republican from New York, said in a statement that "as far as comments being made and comparisons of various tactics and methods between now and World War II, you can make the comparison a little differently and it would be accurate, but it's important to clear up that Hitler did in fact use chemical warfare to murder innocent people".

According to the US Holocaust Memorial Museum, the Nazis experimented with poison gas in late 1939 with the killing of mental patients, which was termed "euthanasia".

Both mobile and stationary gas chambers were later used, with up to 6,000 Jews gassed each day at Auschwitz alone.

The Holocaust Museum did not mention Spicer's comments directly, but sent out a tweet shortly after the briefing that showed graphic footage of bodies US forces found while taking control of the Buchenwald concentration camp.