OPPOSITION leaders have demanded a parliamentary vote before any new military action in Syria as Theresa May called an emergency Cabinet meeting to discuss the growing international crisis.

The Prime Minister summoned her top team to No 10 amid signs she is preparing to join US-led air strikes against Syrian targets after saying "all the indications" were that President Bashar Assad's regime was responsible for an alleged chemical attack on its own people last weekend.

And in a new twist to the unfolding diplomatic drama, US President Donald Trump issued a fresh tweet saying: "Never said when an attack on Syria would take place. Could be very soon or not so soon at all!"

Labour leader Jeremy Corbyn raised the spectre of the Iraq war as he insisted MPs should have their say.

He said: "Parliament must be consulted on this.

"Surely the lessons of Iraq, the lessons that came there from the Chilcott Report, are that there's got to be, there has to be, a proper process of consultation.

"We elect Parliament, we elect members of Parliament. They should have a voice in this. Cabinet on its own should not be making this decision.

"The dangers of bombing now, which could escalate the conflict beyond belief. Just imagine the scenario if an American missile shoots down a Russian plane or vice versa. Where do we go from there?"

Liberal Democrat leader Sir Vince Cable told the BBC: "Parliament can and should be recalled immediately and a vote held on this issue.

"The position is a very dangerous one because of Russian involvement, also because we have an erratic president of the United States."

SNP leader at Westminster Ian Blackford told the BBC: "There is no mandate for the Government to take this action.

"And, I would simply say to the Prime Minister: be very careful, because you do not have a majority in Parliament.

"You are a minority Government, and you need to seek the consent of Parliament before you commit the United Kingdom to any action."

Brexit Secretary David Davis said: "It is a very, very delicate circumstance, and we've got to make this judgment on a very careful, very deliberate, very well thought-through basis, knowing exactly ... how strong the evidence is."

Mr Davis suggested he had changed his mind since he voted against Syrian intervention in 2013 when David Cameron was prime minister.

Outlining the reasons why he voted against, he said: "One was because he (Mr Cameron) hadn't provided the evidence and intelligence that we knew who it was, and secondly because there was not a proper plan which was thought through properly.

"Those two things, I am assured, we're going to answer today."

The Ministry of Defence refused to comment on a report in the Daily Telegraph that Royal Navy submarines had been ordered into range to launch Tomahawk cruise missile strikes as early as Thursday night.

"We don't comment on submarine movements," a spokesman said.

Downing Street would not be drawn on claims that Mrs May was preparing to authorise UK forces to strike against Assad, first seeking a vote in Parliament.

In a development on Thursday morning, Russian media reported Syrian government forces had seized control of the city at the centre of the escalating tensions, Douma, where the attack is said to have taken place.

Kremlin-backed news agency Tass reported a Moscow official saying that Russian military police will be deployed to the city to maintain law and order.

The Organisation for the Prohibition of Chemical Weapons had said it intended to send investigators to Douma to look for evidence of a chemical attack.

Mr Trump's latest intervention came after he previously tweeted that missiles "will be coming".

White House press secretary Sarah Huckabee Sanders insisted no final decisions had been taken and that "all options are on the table".

She made clear that Mr Trump held Assad and his principal backer, Russia, responsible for what happened in the rebel-held town of Douma.

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.

It came almost exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people.

That assault prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base.