US President Donald Trump speaks during a joint news conference with Jordan's King Abdullah (not pictured) in the Rose Garden after their meeting at the White House in Washington, US, April 5, 2017. [Photo/Agencies]

Trump said the attack, which killed at least 70 people, many of them children, "crosses many, many lines", an allusion to his predecessor Barack Obama's threat to topple Assad with airstrikes if he used such weapons. His accusations against Assad put him directly at odds with Moscow, the Syrian's president principal backer.

"I will tell you, what happened yesterday is unacceptable to me," Trump told reporters at a news conference with Jordan's King Abdullah on Wednesday.

"And I will tell you, it's already happened that my attitude toward Syria and Assad has changed very much," though when asked at an earlier meeting whether he was formulating a new policy on Syria, Trump said: "You'll see."

Vice President Mike Pence, when asked whether it was time to renew the call for Assad to be ousted and safe zones be established, told Fox News: "But let me be clear, all options are on the table," without elaborating.

US officials rejected Russia's assertion that Syrian rebels were to blame for the attack.

Trump's comments, which came just a few days after Washington said it was no longer focused on making Assad leave power, suggested a clash between the Kremlin and Trump's White House after initial signals of warmer ties. Trump did not mention Russia in his comments on Wednesday but Secretary of State Rex Tillerson said it was time for Russia to think carefully about its support for Assad.

Pence said the time had come for Moscow to "keep the word that they made to see to the elimination of chemical weapons so that they no longer threaten the people in that country."

Western countries, including the United States, blamed Assad's armed forces for the worst chemical attack in Syria for more than four years.

US intelligence officials, based on a preliminary assessment, said the deaths were most likely caused by sarin nerve gas dropped by Syrian aircraft on the town of Khan Sheikhoun on Tuesday. A senior State Department official saidWashington had not yet ascertained it was sarin.

Moscow offered an alternative explanation that would shield Assad: that the poison gas belonged to rebels and had leaked from an insurgent weapons depot hit by Syrian bombs.

A senior White House official, speaking on condition of anonymity, said the Russian explanation was not credible. "We don't believe it," the official said.