President Donald Trump has said Tuesday's chemical weapons attack on Syrian civilians "crossed many lines" and changed his opinion of the Assad regime.
Speaking at the White House during the visit of King Abdullah of Jordan, Mr Trump described the attack as "horrible, unspeakable" and a "terrible affront to humanity".
Asked if the regime's alleged use of chemical weapons against its own people crossed a "red line" for the new US administration, Mr Trump said: "It crossed a lot of lines for me. When you kill innocent children ... that crosses many, many lines."
Mr Trump has in recent days described any effort to remove President Bashar al-Assad in Syria as secondary to defeating the Isis militant group, and White House press secretary Sean Spicer said the US approach was being driven by a new "reality".
But on Wednesday afternoon, Mr Trump said: "That attack on children yesterday had a big impact on me ... my attitude toward Syria and Assad has changed very much."
Earlier, the US envoy to the UN has said America could be "compelled to take its own action" in Syria if the world body fails to act in the wake of the Idlib attack.
"When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action," US ambassador to the UN Nikki Haley told a Security Council meeting on Syria.
She also said Mr Assad has no incentive to stop using chemical weapons unless Russia stops protecting him.
More than 70 people died in the apparent chemical attack in Khan Sheikhoun, where activists and rescue workers found terrified survivors hiding in shelters near the site of the assault.
A Syrian opposition group said renewed air strikes hit the town a day after the attack, which the Trump administration and others have blamed on the Assad regime and his main patrons, Russia and Iran.
Damascus and Moscow have denied they were behind the attack. Russia's Defence Ministry said the toxic agents were released when a Syrian air strike hit a rebel arsenal, an account Britain dismissed at an emergency UN session called in response to the attack.
British ambassador Matthew Rycroft said the UK had seen nothing that would suggest rebels "have the sort of chemical weapons that are consistent with the symptoms that we saw yesterday".
A resolution drafted by Britain, France, and the US stressed the Syrian government's obligation to provide information about its air operations, including the names of those in command of any helicopter squadrons on the day of the attack.
Richard Gowan, UN expert with the European Council on Foreign Relations, told The Independent if the US "launches punitive strikes on Syria now, the the Russians and Chinese will respond furiously in the Security Council. Equally, if the US does not act at all, Haley will find it hard to put much real pressure on the Russians in future".
Mr Gowan said that many countries at the UN thought the US and Ms Haley would "buddy up" with Russia in the Security Council in order to come to some sort of bargain over Syria. "That notion looks pretty much dead now."
The World Health Organisation and the international medical charity Médecins Sans Frontières said victims of the attack appear to show symptoms consistent with exposure to a nerve agent. In a statement, the agency said "the likelihood of exposure to a chemical attack is amplified by an apparent lack of external injuries reported in cases showing a rapid onset of similar symptoms, including acute respiratory distress as the main cause of death."