Update 8.30pm: Downing Street has said the UK Cabinet agreed the use of chemical weapons in Syria must not go "unchallenged" but gave no immediate details of UK involvement in any military action against the regime of Bashar Assad.
Following a two-hour emergency meeting summoned by Theresa May, No 10 said ministers had agreed Assad had a "track record" of using chemical weapons and that it was "highly likely" he was responsible for the attack on the rebel-held town of Douma on Saturday.
A No 10 statement said: "The Prime Minister said it was a shocking and barbaric act which killed up to 75 people, including children, in the most appalling and inhumane way.
"Cabinet agreed that the Assad regime has a track record of the use of chemical weapons and it is highly likely that the regime is responsible for Saturday's attack.
"The Prime Minister said it was a further example of the erosion of international law in relation to the use of chemical weapons, which was deeply concerning to us all.
"Following a discussion in which every member present made a contribution, Cabinet agreed it was vital that the use of chemical weapons did not go unchallenged."
The Downing Street statement added: "Cabinet agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime.
"Cabinet agreed the Prime Minister should continue to work with allies in the United States and France to co-ordinate an international response."
Update 5.30pm: US President Donald Trump has told reporters that a decision would be made “fairly soon” on their response to a supposed chemical attack in Syria.
Mr Trump will be holding a number of meetings today with the US Security Council and said he will "see what happens".
These remarks follow and earlier tweet where he posted that he "never said when an attack on Syria would take place. Could be very soon or not so soon at all!"
Italy's Prime Minister has announced they will not have any direct role in Western military action against the Syrian government.
Paolo Gentiloni said Italy "will not participate in Syrian military actions" but will "continue to offer logistical support to allied forces".
Meanwhile, UK minsters have begun arriving to Downing Street to attend an emergency cabinet meeting held by Prime Minister Theresa May.
Defence secretary James Mattis has said the US was "still assessing the intelligence" on whether Assad is to blame for the alleged chemical attack in Syria.
Speaking at a congressional committee hearing today, Mr Mattis said they believe there was a chemical attack but they are still "looking for the actual evidence".
"Some things are simply inexcusable, beyond the pale and in the worst interest of not just the chemical weapons convention but of civilisation itself," he said.
Asked what worries him most about possible military action in Syria, Defense Sec. Mattis says, "On a strategic level, it's how do we keep this from escalating out of control, if you get my drift on that." https://t.co/i96bPWCQUP pic.twitter.com/S85SOVIvLK— ABC News (@ABC) April 12, 2018
The first group experts from the international chemical weapons watchdog will arrive in Syria today and tomorrow, Syrian's ambassador to the UN has said.
Investigators from the Organisation for the Prohibition of Chemical Weapons have been gathering information since the alleged chemical attack in Douma, and will undertake a fact-finding mission in the coming days.
#BREAKING First group of OPCW chemical weapons experts will arrive in Syria today: Syria's UN envoy— AFP news agency (@AFP) April 12, 2018
Maria Zakharova, from the Russian Foreign Ministry, said today that the West should "seriously consider" the consequences of threats against Syria.
“Nobody has authorised Western leaders to take on the role of global police - simultaneously investigator, prosecution, judge and executor,” she said during a press briefing.
“Our position is perfectly clear and defined. We are not seeking escalation.”
France has proof that the Syrian government launched chlorine gas attacks, President Emmanuel Macron said.
Mr Macron said France would not tolerate "regimes that think everything is permitted".
Speaking on TF1 television, Mr Macron said "we have proof that chemical weapons were used, at least chlorine" in recent days by Syrian President Bashar Assad's government.
He did not say whether France is planning military action against Assad's government.
Mr Macron said he has been talking regularly this week with US President Donald Trump about the most effective response.
With increasing concerns about a US-Russia proxy war in Syria, Mr Macron insisted that "France will not allow an escalation or something that could damage the stability" of the region.
Chancellor Angela Merkel has say that while it is "obvious" Syria didn't eradicate its chemical arsenal, Germany will not be taking part in any military action.
#BREAKING 'Obvious' that Syria didn't eradicate chemical arsenal: Merkel— AFP news agency (@AFP) April 12, 2018
Mrs Merkel said in Berlin: "Germany will not take part in possible military action - I want to make clear again that there are no decisions - but we see, and support this, that everything is being done to send a signal that this use of chemical weapons is not acceptable."
US President Donald Trump has commented on the US response to an alleged chemical attack in Syria, saying that an attack "could be very soon or not so soon at all".
Mr Trump tweeted this morning that he "never said when an attack on Syria would take place. Could be very soon or not so soon at all!"
Never said when an attack on Syria would take place. Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our “Thank you America?”— Donald J. Trump (@realDonaldTrump) April 12, 2018
He also said, that the US, under his Administration, "has done a great job of ridding the region of ISIS".
"Where is our 'thank you America?'," he asks.
The US Security Council is due to meet later today.
The US has said that all options are on the table in response to the suspected 'chemical attack' in Syria last week.
In a press conference held at the White House last night, spokeswoman Sarah Sanders told reporters that a numver of options have been discussed by President Trump and his security team but no decision has been made as yet.
"The president and his national security team met today. That meeting was chaired by the vice president and they discussed a lot of options," White House Press Sec. Sarah Sanders says on Syria talks. https://t.co/YLYOTpuPzo pic.twitter.com/oVMgsjHSII— NBC News (@NBCNews) April 12, 2018
Theresa May is to chair an emergency Cabinet meeting to discuss the latest crisis in Syria amid signs she is preparing to join US-led air strikes against the regime of President Bashar Assad.
The British Prime Minister summoned ministers to No 10 after saying "all the indications" were that the regime was responsible for an alleged chemical attack on its own people last weekend.
The UK's 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.
Separately, Downing Street would not be drawn on claims Mrs May was preparing to authorise UK forces to strike against Assad with first seeking vote in Parliament.
In a development this 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 intends to send investigators to Douma to look for any evidence of a chemical attack.
"Today saw a landmark event in Syria's history. A state flag hoisted on the Douma building heralded control over this settlement and, hence, over entire eastern Ghouta," Yuri Yevtushenko, chief of Russia's centre for reconciliation of the warring parties in Syria, told Tass.
An airline has announced it is stopping flights to Beirut amid safety fears as the prospect of US-led military action over chemical weapons in Syria grows.
Kuwait Airways said concerns for aircraft in the skies around Lebanon had prompted it to cancel flights to the country's capital from Thursday.
The carrier said it had received a warning from Cypriot authorities over operations in the area.
The move came after Middle East Airlines, Lebanon's national airline, said it was rerouting some of its flights "due to the recent security situation between US and Syria".
Yesterday, European aviation authorities issued a warning to airlines operating in the Eastern Mediterranean, which includes pilots flying from the UK to Cyprus, of possible military action.
The "rapid alert" told crews that "due consideration needs to be taken" of possible air-to-ground strikes or cruise missiles into Syria "within the next 72 hours".
The notification from the European Aviation Security Agency (Easa) covers the Eastern Mediterranean/Nicosia flight information region, which includes airspace over Cyprus.
Most carriers already avoid Syrian airspace due to previous warnings from aviation regulators in the UK and other countries, but the Easa alert could affect flights in neighbouring regions.
In July 2014, Malaysia Airlines Flight 17 was shot down over conflict-torn eastern Ukraine, killing 298 passengers and crew.
An international investigation found the missile used was taken into Ukraine from Russia, but the latter denied any involvement.
UN Secretary-General Antonio Guterres has stressed the need to prevent the situation in Syria from "spiralling out of control".
Guterres expressed regret on Wednesday that the UN Security Council had been unable to reach agreement on the issue of chemical weapons in Syria.
He said he had called the ambassadors of the five veto-wielding permanent council nations - the US, Russia, China, Britain and France - "to reiterate my deep concern about the risks of the current impasse".
He also renewed his "outrage" at reports of continued chemical weapons use in Syria.
In Australia, prime minister Malcolm Turnbull condemned the suspected chemical weapon attack on Douma.
Speaking to reporters in Melbourne, he said: "This is a shocking attack yet again, a war crime by the Syrian government.
"Russia has enormous influence with the Syrian regime and it should bring that influence to bear to stop the use of chemical weapons.
"We cannot accept the use of chemical weapons, whether it is in Syria by the Syrian regime or on a park bench in Salisbury, England, with the use of a nerve agent by Russian agents to seek to kill the Skripals."
- Digital Desk and Press Association