The Prime Minister summoned ministers to No 10 after saying “all the indications” were that the regime was responsible for a chemical attack on its own people last weekend.
Home Secretary Amber Rudd is one of the members of the Cabinet arrived in Downing Street.
Defence Secretary Gavin Williamson and Culture Secretary Matt Hancock have also arrived.
Royal Navy submarines have already been ordered into range to launch Tomahawk cruise missile strikes as early as Thursday night, according to reports.
Mrs May faces growing impatience from Washington, where President Donald Trump tweeted the missiles “will be coming”.
White House press secretary Sarah Huckabee Sanders later insisted no final decisions had been taken and that “all options are on the table”.
But she made clear that Mr Trump held Assad and his principal backer, Russia, responsible for what happened in the rebel-held town of Douma.
She said: ”The president holds Syria and Russia responsible for this chemical weapons attack.”
At home, Mrs May has been warned to wait for unequivocal proof of a chemical attack by the Assad regime before committing British forces to retaliatory action.
Labour leader Jeremy Corbyn joined other opposition parties – as well as some Conservative backbenchers – in insisting MPs must be have a say on any British involvement in military action.
Speaking during a visit to Birmingham on Wednesday, Mrs May sidestepped questions as to whether she was prepared to give MPs a vote.
She said that she would continue to work with Britain’s “closest allies” – the US and France – to ensure that those responsible for the “barbaric” attack were held to account.
“We have been working to get an understanding of what happened on the ground. We are rapidly reaching that understanding,” she said.
“All the indications are that the Syrian regime was responsible and we will be working with our closest allies on how we can ensure that those who are responsible are held to account.
“The continued use of chemical weapons cannot go unchallenged.”
Since MPs do not return from their Easter break until next Monday, Mrs May could decide not to wait and take military action without parliamentary approval.
But Tory former chancellor Ken Clarke protested spoke out against the possibility, calling for a parliamentary vote on any military action.
“To say that Parliament is just sidelined before you take such a serious decision is a very retrograde step.
“It makes parliamentary accountability fairly pathetic.”
Her comments came as both Washington and Moscow ratcheted up the growing war of words between the two sides.
Russia’s ambassador to Lebanon, Alexander Zasypkinhad, warned they would shoot down US rockets and “even the sources that launched the missiles” – suggesting they could hit American aircraft or warships.
That prompted Mr Trump to tweet: “Russia vows to shoot down any and all missiles fired at Syria.
“Get ready, Russia, because they will be coming, nice and new and ‘smart’. You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”
The attack in Douma happened late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
Syrian opposition activists and rescuers said poison gas was used on the rebel-held town near the capital, an allegation strongly denied by the Assad government.
Source : EXPRESS