Donald Trump has stated "something should happen" in retaliation for the chemical weapons strike by the Assad regime as defence officials drew up options for military intervention in Syria.
The US president suggested Bashar al-Assad may have to leave power, while his secretary of state, Rex Tillerson, made a complete reversal of his position a week ago and said there was "no role" for Mr Assad in governing Syria.
The chemical weapons attack on a village in Syria's north-east Idlib province left 86 people dead including at least 30 children.
Intelligence officials in both Britain and America said that the deadly gas sarin was used and that the strike was undoubtedly carried out by Assad regime war planes.
"I think what Assad did is terrible," Mr Trump said on board Air Force One. "I think what happened in Syria is a disgrace to humanity and he's there, and I guess he's running things, so something should happen."
It is Mr Trump's first major foreign policy test and James Mattis, the defence secretary, flew to Florida on Thursday evening to brief the president on America's military options, one of which would be cruise missile strikes from Navy ships.
Mr Tillerson said that "steps are under way" to work with international partners to build a coalition to remove Mr Assad and promised a “serious response” to the Idlib attack from the United States.
However, America's European allies urged caution and warned of the risk of becoming embroiled in another costly war in the region.
Boris Johnson, the Foreign Secretary, said that passing a United Nations resolution should be the top priority - not rushing into a new confrontation.
“It is very important to try first to get out a new UN resolution,” Mr Johnson said.
Jean-Marc Ayrault, the French foreign minister, said: “(The first stage) is not to go in ourselves, under the pretext that the US president may have a rush of blood to the head, and get onto a war footing.”
He added that the US response on Syria was still unclear, and that he was getting mixed messages from Mr Tillerson and Mr Mattis. "They're not saying the same thing," he said.
Any attempt by Washington to target the regime directly could see it come into confrontation with its main backer, Russia, which it will be looking to avoid.
Russia on Thursday challenged Mr Trump to set out his strategy on Syria.
When asked if Moscow would reconsider its backing for Assad, Russia's foreign ministry instead challenged the US to show its cards.
"Russia's approach to Assad is clear. He is the legal president of an independent state. What is the US approach?" ministry spokeswoman Maria Zakharova said.
The US faced a similar dilemma after the last major chemical attack in Syria in 2013, when Barack Obama prepared plans to strike targets with sea-launched cruise missiles.
Mr Obama's plans were thrown into disarray after British MPs rejected a motion designed to pave the way to authorising the UK's participation in military action.
The White House decided not to take the unpalatable option of unilateral action without the support of its main ally. The opposition says allies’ inaction has since allowed Mr Assad to act with impunity.
Two US military officials told NBC News that the military had seen Syrian fixed-wing aircraft drop the bombs after the planes were caught on radar.
The Syrian government continued yesterday to deny its role in Tuesday's attack, repeating Russian claims an air strike hit a rebel warehouse storing chemical agents.
"I stress to you once again that our army has never used chemical weapons and will not use chemical weapons," Walid Muallem, the country’s foreign ministry, said on state TV. "Not only against our civilians, our people, but also will not use chemical weapons against the terrorists who are attacking our civilians with their mortar shells and killing civilians."
Preliminary postmortem tests conducted by officials from the World Health Organisation on victims treated in Turkey indicated on Thursday that the deadly nerve agent sarin was used in the attack, according to the country’s justice minister.
He said that results pointed to government responsibility, without elaborating.
Opposition activists on Thursday shared pictures on social media of children from Idlib protesting with duct tape over the mouths, signifying the world’s silence and inaction over the atrocities taking place in Syria.
One girl holds a sign reading “Friends of Syria” next to seven children bearing the flags of countries which support the rebels looking over “bodies” on the ground.