A day after warning starkly that "missiles will be coming", Trump in another early morning tweet storm wrote: "Never said when an attack on Syria would take place".
Liberal Democrat leader Sir Vince Cable said President al Assad had crossed "a clear red line", but also demanded the Government "present the objectives of any proposed action to Parliament".
Russian lawmakers have warned the United States that Moscow would view an airstrike on Syria as a war crime and that it could trigger a direct U.S-Russian military clash.
He added: "Russia vows to shoot down any and all missiles fired at Syria".
Russian Federation needs us to help with their economy, something that would be very easy to do, and we need all nations to work together.
May will hold an emergency meeting with her Cabinet on Thursday after stating that "all the indications" reaffirm reports that the chemical strike in Douma this week was carried out by Bashar al-Assad's regime.
President Emmanuel Macron said France, the USA and Britain will decide how to respond in the coming days.
A pinpoint US strike on Syrian targets that does not harm Russian personnel "will allow Trump to say that the Assad regime has paid a heavy price ... and Russia in its turn will be able to limit itself to ringing statements", said Fyodor Lukyanov, the head of the Council for Foreign and Defense Policies, an association of top Russian political and security experts.
Macron has said that any French strikes against the Syrian regime would target chemical facilities, and a British government official told CNN on Wednesday that preparations are being made by the United Kingdom for possible action but cautioned the political decision has not yet been made. The airline announced it would suspend flights to the Lebanese capital due to fears of a USA missile attack on Syria.
"This line exists and it's in an active state, ' spokesman Dmitry Peskov said, adding that the line meant to prevent accidental clashes 'is used by both sides".
Asked if he had seen enough evidence to blame Assad, Mattis said: "We're still working on this". Trump responded by launching Navy cruise missiles at a Syrian airfield.
"Either of those actions are created to interfere with an investigation that may implicate the President", House Intelligence Committee ranking member Adam Schiff said on CNN's "The Situation Room".
Russian Federation is Mr Assad's most powerful ally, and its devastating air power has helped him wrest back large areas of territory from rebels since 2015. So the three countries are discussing a range of options, including preventing Assad from conducting future attacks by striking military capabilities involved in carrying out such attack, the official said.
As tensions mounted over a face-off with Damascus-ally Russian Federation, opponents of unilateral United States action called an emergency closed-door meeting of the UN Security Council for Thursday. "Now worldwide chemical weapons inspectors are demanding safe and unhindered passage to and from Douma to determine whether globally banned munitions were used".