White House Press Secretary Sean Spicer speaks during a press briefing at the White House in Washington, US, April 11, 2017. [Photo/Agencies]

Trump, who has faced criticism for lacking a broader strategy to deal with the Syria crisis, insisted he has no plans to "go into" the war-torn country.

Senior White House officials, briefing reporters on condition of anonymity, said Assad's government carried out the April 4 sarin nerve gas attack on civilians in Syria's Idlib province, killing 87 people including many children, to put pressure on rebels making advances in the area.

Russia has defended the Syrian leader against US allegations that his forces carried out the attack, saying there is no evidence. Russia has blamed Syrian rebels.

"It's clear that the Russians are trying to cover up what happened there," one White House official said.

White House spokesman Sean Spicer later told reporters that the facts backed up the US version of events. "Russia is on an island when it comes to its support of Syria or its lack of, frankly, acknowledgment of what happened," he told reporters.

But at the same briefing Spicer drew criticism after he sought to underscore the ghastliness of the gas attack by saying, "You had someone as despicable as Hitler who didn't even sink to using chemical weapons." Nazi Germany used gas chambers to kill millions of Jews during the Holocaust.

Spicer later apologized and said he should not have made the comparison. "It was a mistake. I shouldn't have done it and I won't do it again," Spicer told CNN in an interview. "It was inappropriate and insensitive."

The White House officials said Russia has frequently offered multiple, conflicting accounts of Syrian government aggression including the incident in the village of Khan Sheikhoun to sow doubt within the international community.

The United States launched 59 cruise missiles at a Syrian airfield on Thursday to retaliate after the attack. The strikes thrust the administration of President Donald Trump, who came to power in January calling for warmer ties with Russia, into confrontation with Moscow.

Trump's ambassador to the United Nations, Nikki Haley, said on Tuesday she thought Russia knew about the chemical attack inadvance. "They didn't look shocked. They didn't look surprised.

They were so quick to defend. And then the evidence comes out, and we see exactly what it is and we know exactly what the environment was. Then you realize," she said on CNN.

US intelligence indicates that the chemical agent in the attack was delivered by Syrian Su-22 aircraft that took off from the Shayrat airfield, according to a White House report given to reporters.

In a four-page document, the White House sought to rebut many of Moscow's claims about the circumstances of the attack.

It said the Syrian planes were in the vicinity of Khan Sheikhoun about 20 minutes before the attack and left shortly afterward.

"Additionally, our information indicates personnel historically associated with Syria's chemical weapons program were at Shayrat airfield in late March making preparations for an upcoming attack in northern Syria, and they were present at the airfield on the day of the attack," the report said.

Washington wants Russia to stop supporting Assad, who has been fighting a six-year-long civil war against mostly Sunni Muslim rebels, also with the backing of Shi'ite Muslim Iran.