British Prime Minister Theresa May's senior ministers agreed on the need for action against Syria at a Cabinet meeting on Thursday, but Downing Street did not specify what measures the United Kingdom would take. "Could be very soon or not so soon at all!" he wrote.
That assessment appeared to echo French President Emmanuel Macron who said they had "proof" that "at least chlorine" was used in the attack by the regime.
"The Prime Minister said it was a further example of the erosion of global law in relation to the use of chemical weapons, which was deeply concerning to us all".
"It is worth noting, however, that even if military action is taken before MPs return from their Easter recess, May will nearly certainly have to explain her response to parliament and seek its approval for any longer-term engagement", he adds.
Britain is now part of a US-led coalition conducting air strikes against jihadists from the Islamic State (IS) group in Iraq and Syria, and has conducted more than 1,700 such attacks.
Compared with handling the Salisbury attack, responding to worldwide calls to retaliate against the Assad regime is a far more challenging undertaking for Mrs May, not least because it is the first time she has been asked to involve Britain in military action since taking office.
But without a Commons vote, she alone will have to take full responsibility for attacking Syria, a burden that could come back to haunt her.
She said the Douma incident was a "shocking and barbaric act" which killed dozens of innocent people.
Mrs May could find herself in difficulty if, as seems likely, she decides to authorise military action without first receiving parliamentary backing.
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On Wednesday, opposition Labour Party leader Jeremy Corbyn urged May to recall Parliament - which is on recess - to hold a vote before taking any action.
Later, Mrs May's office said she had spoken with Mr Trump by telephone, and that the two had agreed it was vital to challenge Mr Assad's use of chemical weapons, and that they would continue to work closely together to do so.
"They should have a voice in this".
Ali Akbar Velayati, a top adviser to Iran's Supreme Leader Ali Khamenei, said the Western threats were "based on lies" about the poison gas assault, after meeting Mr Assad.
Anti-conflict coalition Stop the War called on Britons to lobby their lawmakers to prevent an "escalation of the war" and planned a Friday protest outside Downing Street.
Conservative former London mayoral candidate Zac Goldsmith tweeted: "We need a clear response to the Syrian chemical outrage". "Where do we go from there?"
SNP leader at Westminster Ian Blackford told the BBC: "There is no mandate for the Government to take this action". Inspectors with the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, are due to investigate the incident.
The Ministry of Defence refused to comment yesterday on a report that Royal Navy submarines had been ordered into range to potentially launch Tomahawk cruise missile strikes as early as last night.
"But also it's a very, very delicate circumstance and we've got to make this judgment on a very careful, very deliberate, very well thought-through basis".