US President Donald Trump has promised a “forceful” response to the alleged chemical weapons attack in Syria.
Russia has denied the attack took place or that Moscow or that the Syrian military was involved.
President Bashar al Assad's regime in Syria, which is backed by Russia, has also denied responsibility for the attack.
At least 70 people, including women and children, are reported to have died and a further 500 injured in the chemical attack on the rebel-held city of Douma.
The Syrian American Medical Society described patients foaming at the mouth and warned that victims had suffered corneal burns and smelled of a "chlorine-like odour."
There has been no independent verification of the claims - made by the White Helmets rescue service and other medical groups - as international groups have been unable to enter the enclave as yet.
The below video contains graphic and upsetting images:
Another Video showing cases of suffocation among civilians, mostly children and women, following the chemical attack against the civilians in #Douma city. #AssadHitsDoumaWithChemicals pic.twitter.com/ItEpVLq5zM
— The White Helmets (@SyriaCivilDef) April 8, 2018
At an emergency meeting of the United Nations Security Council, US Ambassador Nikki Haley said Moscow had the "blood of Syrian children" on its hands.
At a Cabinet meeting last night, President Trump said the US has “a lot of options militarily” adding that a response will be decided.
He said his administration will make "major decisions" over the next 24 to 48 hours in response to the "barbaric" and "heinous" attack.
— Department of State (@StateDept) April 9, 2018
On Newstalk breakfast this morning, international relations expert Scott Lucas said American intervention in Syria is almost inevitable:
“I think they have got to act because of Trump’s tweet on Sunday that promised action, I think they are in a corner,” he said.
“I think they will strike an Assad regime military facility as early as later this week.
“But I don’t think the Us and other countries will go with what is needed which is to actually establish protective zones to prevent further bombing.”
Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...
— Donald J. Trump (@realDonaldTrump) April 8, 2018
....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!
— Donald J. Trump (@realDonaldTrump) April 8, 2018
Russia however, has warned the US of "grave repercussions" if it strikes against the Syrian regime.
At a meeting of the United Nations Security Council in New York, Russia's envoy Vassily Nebenzia insisted there was "no chemical weapons attack."
"Through the relevant channels we already conveyed to the US that armed force under mendacious pretext against Syria - where, at the request of the legitimate government of a country, Russian troops have been deployed - could lead to grave repercussions," he said.
Russian Ambassador to the United Nations Vassily Nebenzia during the UN Security Council meeting on Syria, 09-04-2018. Image: Li Muzi/Xinhua News Agency/PA Images
The US has said it will call for a new UN Security Council vote on a fresh inquiry into the use of chemical weapons in Syria.
But Mr Trump earlier declared the US will "know the answers quite soon" over the Douma attack, as he suggested those to blame could be Syria, Russia, Iran or "all of them together."
The US President also warned "nothing is off the table" when asked if he would consider military action.
Mr Nebenzia claimed Russia is being "unpardonably threatened" by Western powers with a tone that "has gone beyond the threshold of what is acceptable, even during the Cold War."
He also claimed experts from the international chemical weapons watchdog should fly to Syria to probe the allegations, as he offered them protection in their work via Syrian authorities and Russian troops in the country.
US Ambassador to the UN Nikki Haley addresses the UN Security Council meeting on Syria, 09-04-2018. Image: Li Muzi/Xinhua News Agency/PA Images
However, Nikki Haley, the US ambassador to the UN, insisted America "will respond" whether the UN Security Council acts or not.
She said: "We are on the edge of a dangerous precipice. The great evil of chemical weapons use that once unified the world in opposition is on the verge of becoming the new normal.
"The international community must not let this happen. We have reached the moment when the world must see justice done.
"History will record this as the moment when the Security Council either discharged its duty, or demonstrated its utter and complete failure to protect the people of Syria.
"Either way, the United States will respond."
Security Council Veto
As a permanent member of the UN Security Council, Russia can veto any substantive resolution.
Mrs Haley added "the monster" responsible for chemical weapons attacks in Syria "has no conscience", as she told Moscow its "hands are all covered in the blood of Syria children".
After the Syrian regime used sarin nerve agent during an attack in Khan Sheikhoun in April last year, Mr Trump launched retaliatory missile strikes against a Syrian government airbase.
Additional reporting from IRN ...