Here is a recap of events since April 7:

- 'Poisonous chlorine gas'-

On April 7 the Syrian American Medical Society (SAMS) and Syria's White Helmets jointly say more than 40 people have died in a "poisonous chlorine gas" in Douma, the last remaining opposition-held town in Eastern Ghouta, once the main rebel stronghold outside Damascus.

The statement said "more than 500 cases" showed "symptoms indicative of exposure to a chemical agent".

The Syrian Observatory for Human Rights, a Britain-based monitor, does not confirm a chemical attack.

However, it reports at least 70 cases of respiratory difficulties among civilians trapped underground or in poorly-ventilated rooms after regime air strikes, specifying 11 people had died.

- 'Big price to pay' -

The following day Trump tweets a warning aimed at Assad and his allies that there will be a "big price to pay" after what he calls a "mindless CHEMICAL attack". He hits out at Russia and Iran over their backing for "Animal Assad".

Moscow warns Washington against carrying out a "military intervention on fabricated pretexts", which "could have the most dire consequences".

Damascus denies having carried out a chemical attack, denouncing the accusations as an "unconvincing broken record".

The EU says that signs suggest the regime carried out the attack and urges Russia and Iran to help prevent another one.

- Russia and Iran -

A missile attack on April 9 targets Syria's T-4 military airbase in the central province of Homs.

Iran, Syria and Russia blame Israel for the strike that kills up to 14 people, according to the Observatory. Paris and Washington deny responsibility.

Russia says its specialists have found no trace of a chemical attack on Douma.

At a UN Security Council meeting US Ambassador Nikki Haley warns Washington is determined to "see that the monster who dropped chemical weapons on the Syrian people is held to account".

The USS Donald Cook -- a guided-missile destroyer -- leaves Cyprus, moving within easy striking range of Syria.

- 'State of emergency' -

On April 10 the Syrian army puts its military positions, including airports and all bases, on alert for three days, the Observatory reports.

France says "there will be a response" if the "red line" of chemical weapons use has been crossed.

Intelligence shared by the US and France had "in theory" confirmed the use of banned substances in the attack, it adds.

The global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, announces it will "shortly" deploy a fact-finding team to Douma.

UN Secretary General Antonio Guterres calls for an "impartial" investigation, which "should be granted full access".

Trump cancels travel plans for the end of the week.

France says it will decide "in the coming days" on its response, in conjunction with Washington and London.

"Our decision will not target allies of the regime or attack anyone but rather attack the regime's chemical capabilities," says Macron, insisting he does "not want an escalation".

At the UN, Russia vetoes a US-drafted Security Council resolution that would have set up an investigation into chemical weapons use in Syria.

The body also votes twice on draft texts put forward by Russia, but both fail.

- Missiles 'will be coming' -

On April 11 the Kremlin warns against moves that "could destabilise the already fragile situation in the region".

In an escalating war of words with Moscow, Trump takes to Twitter. Ties with Russia are worse than during the Cold War, he says.

"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!"

US Defense Secretary Jim Mattis says the Pentagon is ready to provide options for a strike but notes the US and its allies are still assessing intelligence on the alleged strike.

The Kremlin announces the deployment of its military police in Douma as of Thursday.

Bolivia -- a strong supporter of Russia -- requests a closed-door UN Security Council meeting to be held the same day on the threat of military action against Syria.