An emergency meeting of her "War Cabinet" yesterday accepted it was "highly likely" the dictator's regime was responsible for the atrocity that killed around 75 civilians and agreed the outrage could not go "unchallenged".
But amid signs of misgivings among some senior Tories, the meeting left the issue of whether British Armed Forces will join an imminent US-led strike without parliamentary approval was left unresolved.
Instead, ministers called for further diplomatic efforts with the US and France to "co-ordinate an international response" to the crisis.
Ministers, summoned to Number 10 for the emergency session during the parliamentary Easter break, spent more than two hours discussing the options for backing US President Trump's threatened attack on Syrian government forces.
President Trump yesterday went some way towards reducing expectations of an imminent attack.
"Never said when an attack on Syria would take place. Could be very soon or not so soon at all!" he said on Twitter.
The remark contrasted with his warning to Russia to "get ready" for an attack on its ally Assad earlier this week. He had claimed missiles were "coming" and would be "nice and new and smart".
The President was due to discuss attack plans with military chiefs on his National Security Council last night.
"We're having a number of meetings today, we'll see what happens," the President said. "Now we have to make some decisions, so they'll be made fairly soon."
US Defense Secretary Jim Mattis, speaking at a congressional committee hearing, said: "We have not yet made any decision to launch military attacks into Syria."
He added: "When I leave here, I go to a meeting where the National Security Council will be meeting on this and we will take forward various options to the President."
French President Emmanuel Macron yesterday claimed to have "proof" that Syrian government forces had deployed chemical weapons.
"We have proof that last week chemical weapons, at least chlorine, were used by the regime of Bashar al-Assad," he said.
Asked in a TV interview if French military units would join strikes on Syria, he said: "We will need to take decisions in due course, when we judge it most useful and effective.
"Regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act."
In Downing Street, Cabinet ministers remained tight lipped as they arrived for the meeting at 3.30pm and left shortly before six.
While Foreign Secretary Boris Johnson and Environment Secretary Michael Gove were thought to be among ministers backing the case for reprisals against Assad, some of their colleagues were believed to have misgivings about the political dangers of joining a US-led conflict without parliamentary approval.
Every Cabinet minister contributed to yesterday's discussion about the options.
Mrs May told the meeting the attack against innocent civilians in Douma, Syria, on Saturday was "a shocking and barbaric act which killed up to 75 people, including children, in the most appalling and inhumane way."