Update - 4.01pm: The Russian military has claimed an alleged chemical attack in Syria was staged and directed by Britain.
Volunteer first responders and activists said a chemical attack by the Syrian government killed more than 40 people in the town of Douma, which drew international outrage and prompted Washington and its allies to consider a military response.
Moscow warned against any strikes and threatened to retaliate.
Russian Defence Ministry spokesman, Major General Igor Konashenkov, released statements by medics from Douma's hospital who said a group of people toting video cameras entered the hospital, shouting that its patients had been struck with chemical weapons and causing panic.
The medics said none of the patients were hurt by chemicals.
Maj. Gen. Konashenkov said Britain was "directly involved in the provocation".
The US ambassador to the United Nations said Donald Trump "has not yet made a decision about possible actions in Syria".
Nikki Haley told an emergency meeting of the UN Security Council called by Russia that should the US and its allies decide to act in Syria it will be to defend "a bedrock international norm that benefits all nations" - the prohibition on the use of chemical weapons.
She said "the United States estimates that (President Bashar) Assad has used chemical weapons in the Syrian war at least 50 times".
On the suspected poison gas attack on Douma, Ms Haley said: "We know who did this. Our allies know who did this. Russia can complain all it wants about fake news, but no one is buying its lies and its cover-ups."
1.56pm: Russia claims Syria was warned that rebels would carry out chemical attack
Russia's ambassador to the UK Alexander Yakovenko has told a London press conference: "The Syrian government had been repeatedly warning for at least a month that rebels prepared to stage a provocation with chemical weapons in this very area."
Mr Yakovenko said that international inspectors were expected in Syria on Saturday, adding: "But I believe that any delay may be used to carry out military strikes to cover up all the evidence, or lack thereof, on the ground."
Mr Yakovenko, who showed footage of former British prime minister Tony Blair apologising for intelligence mistakes in the run-up to the Iraq war at the event, said the US and its allies had provided "no tangible proof" to back up claims the Syrian government was responsible for the attack.
He added: "We are witnessing very dangerous developments in Syria. The current US politics, supported almost mechanically by France and the UK, is becoming a threat to the peace and security in the region and beyond."
Mr Yakovenko said Russia was sending an official request to the UK today under article nine of the Chemical Weapons Convention seeking "clarification and information" regarding questions and concerns Moscow has.
The Russian ambassador said Ms Skripal was free to live where she wants, stating: "This is her decision. She can choose any country to stay or leave. We want to investigate this case and we definitely want to hear her story."
It comes after the Russian Embassy in the UK said it was "concerned about the decision of the Cabinet to "take action" amid the reports on the possible UK participation in the military operation against Syria".
It added: The preparations are being made despite lack of evidence as regards to what happened on the ground in British interpretation. At the request of the Syrian Government, the OPCW sent a fact-finding team to Douma. The experts will arrive tomorrow. Syrian authorities will provide all the relevant security guarantees. Russia is ready to assist in ensuring its safety. We are interested in seeing independent experts there, so they can make all the necessary tests without delay.
"The conditions on the ground, now that the Russian military police is present in Douma, are appropriate for conducting an investigation of the alleged incident. Russian services for chemical and radiological security visited the suspected site of the incident and did not find any traces of chemical substances. No persons treated for chemical poisoning were found in local hospitals.
"We took note of the recent statement of the UK Permanent Representative to the UN Karen Pierce, calling the idea of an investigation "an offer worth pursuing". What is striking is that the UK statements in favour of an independent investigation clearly contradict those automatically laying the blame on the Syrian government.
"At the same time we receive worrying reports about London pressing ahead with preparations for the military action against Syria.
"Military strikes may be used to cover up all the evidence, or lack thereof, on the ground. Such a decision, if it's taken, in violation of the international law and the Charter of the United Nations may well mark the latest in the series of the reckless military adventures by the UK.
"The infamous aggression against Iraq in 2003 immediately comes to mind, when the then-Prime Minister Tony Blair deliberately misinformed the Parliament and the public. It is well known how it ended - with Chilcot inquiry delivering a condemnatory verdict. It's essential to avoid any steps which could escalate the tensions."
11.55am: Syria chemical attack 'fabrication by foreign power', says Russia
Russia's foreign minister has claimed a suspected chemical attack in the Syrian town of Douma last weekend was fabricated with the help of an unspecified foreign intelligence agency.
Sergey Lavrov said Russian experts have inspected the site of the alleged attack in Douma, just east of Damascus, and found no trace of chemical weapons.
He said Moscow has "irrefutable information that it was another fabrication".
Mr Lavrov, speaking to reporters in Moscow, said: "Intelligence agencies of a state that is now striving to spearhead a Russo-phobic campaign were involved in that fabrication."
He did not elaborate or name the state.
The attack has drawn international outrage and prompted the US and its allies to consider a military strike on Syria, something Moscow has strongly warned against.
9.27am: Jeremy Corbyn accuses Theresa May of 'waiting for instructions' from US on Syria
Britain's Labour leader Jeremy Corbyn has accused the British Government of 'waiting for instructions' from US president Donald Trump on what to do over Syria.
It comes after Theresa May and Donald Trump agreed that the use of chemical weapons must not go unchallenged after the British Prime Minister won the backing of her Cabinet for action to prevent their further use in Syria.
The two leaders also said there was a need "to deter the further use of chemical weapons by the Assad regime", as they pledged to work together on the international response to the suspected chemical weapons attack.
He said: "Further UK military intervention in Syria's appalling multi-sided war risks escalating an already devastating conflict.
"The Government appears to be waiting for instructions from President Donald Trump on how to proceed. But the US administration is giving alarmingly contradictory signals.
"Even US defence secretary James Mattis has said we 'don't have evidence' and warned further military action could 'escalate out of control'."
Mr Corbyn added: "Ministers should take their proposals, such as they are, to parliament. And Britain should press for an independent UN-led investigation of last weekend's horrific chemical weapons attack so that those responsible can be held to account.
"Rather than further military action, what is urgently needed is a coordinated international drive to achieve a ceasefire and a negotiated settlement under UN auspices. The humanitarian priority must be to halt the killing on all sides.
"The need to restart genuine negotiations for peace and an inclusive political settlement of the Syrian conflict, including the withdrawal of all foreign forces, could not be more urgent. We must do everything we can, no matter how challenging, to bring that about."
7.17am: Russia warns West of 'dangerous escalation' in Syria
A team of inspectors from the international chemical weapons watchdog is due to begin an investigation into a suspected chemical weapons attack near the Syrian capital Damascus.
It comes amid Western threats of retaliation and Russian warnings of the potential for "a dangerous escalation" in the area.
Speaking at the United Nations on Thursday, Russia's UN ambassador, Vassily Nebenzia, said the top priority had to be to avert a wider war, and he didn't rule out the possibility of a US-Russia conflict.
Speaking to reporters after a closed emergency meeting of the UN Security Council, Mr Nebenzia said Russia was very concerned with "the dangerous escalation" of the situation and "aggressive policies" and preparations that some governments were making - a clear reference to the Trump administration and its allies.
"We hope that there will be no point of no return - that the US and their allies will refrain from military action against a sovereign state," Mr Nebenzia said, adding that "the danger of escalation is higher than simply Syria."
The Security Council scheduled another emergency meeting for this morning at Russia's request.
The fact-finding mission from the Organisation for the Prohibition of Chemical Weapons was expected to head to Douma, where the suspected attack took place and where Russia said rebels had now capitulated to government control.
The Syrian government said it would facilitate the mission's investigation, which is to start on Saturday.
In case you missed it: In response to media queries, the Spokesperson for the Organisation for the Prohibition of Chemical Weapons (OPCW) confirms that the #OPCW Fact-Finding Mission (FFM) team is on its way to #Syria and will start its work as of Saturday 14 April 2018. pic.twitter.com/IZUIpg2DsZ— OPCW (@OPCW) April 12, 2018
Syria and its ally, Russia, deny any such attack, which activists say killed more than 43 people last weekend.
The imminent arrival of the chemical weapons inspectors came as rebels in Douma surrendered their weapons and left the town for opposition-held areas in the north.
Russia's military said on Thursday that Douma was now under full control of the Syrian government after a Russian-mediated deal secured the evacuation of the rebels and thousands of civilians after it was recaptured by Syrian forces.
Douma and the sprawling eastern Ghouta region near the capital, Damascus, had been under rebel control since 2012 and was a thorn in the side of President Bashar Assad's government, threatening his seat of power with missiles and potential advances for years.
The government's capture of Douma, the last town held by the rebels in eastern Ghouta, marked a major victory for Assad.
Hamza Bayraqdar, spokesman for Jaysh al-Islam, the main rebel group that once controlled Douma, said his fighters had all evacuated. They handed over their heavy and medium weapons, as well as maps of land mines and the tunnels they dug, according to Syrian state media.
Douma and the rest of eastern Ghouta had been a significant rebel stronghold throughout Syria's civil war and its surrender came after years of siege by Assad's troops and a months-long military offensive.
It followed weeks of negotiations mediated by Russia that repeatedly were derailed.
A truce collapsed last week and the Syrian government pressed ahead with its military offensive.
Then came the suspected chemical attack in Douma, followed by international condemnation and threats of military action.
Amid conflicting tweets about the timing of any retaliation, US president Donald Trump said on Thursday that an attack on Syria could take place "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
On Capitol Hill, US defence secretary Jim Mattis said the National Security Council would be meeting later on Thursday to present Mr Trump with various options, adding that he could not talk about any military plans because an attack "is not yet in the offing".
U.S., allies "still assessing the intelligence" needed to prove Assad regime conducted recent alleged chemical attack in Syria, Defense Sec. James Mattis says. https://t.co/dOrNLzNLEv pic.twitter.com/52jEayDBkA— ABC News (@ABC) April 12, 2018
- Digital Desk and Press Association