The United Nations Security Council will meet later to discuss the suspected chemical weapons attack in eastern Syria.
At least 70 people, including women and children, are reported to have died and a further 500 injured in the rebel-held city of Douma.
Donald Trump has warned Russia and Iran there will be a "big price to pay" for backing the Syrian regime after a suspected chemical attack.
It comes as US President Donald Trump condemned as "mindless" and "SICK" reports of the alleged chlorine gas attack in eastern Ghouta.
French President Emmanuel Macron also vowed a "strong, joint response" during a phone conversation, the White House said on Sunday evening.
It added that the regime of Syrian President Bashar al Assad "must be held accountable for its continued human rights abuses".
There has been no independent verification of the claims - made by the White Helmets rescue service and other opposition-linked medical relief groups - of a chemical attack.
However, the European Union has said the "evidence points toward yet another chemical attack".
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
Nine countries have now called for an emergency meeting of the UN Security Council to discuss the reports: the UK, France, the US, Poland, Netherlands, Sweden, Kuwait, Peru and Cote d'Ivore. The meeting will take place later on Monday.
UK, France, US, Poland, Netherlands, Sweden, Kuwait, Peru and Cote d’Ivore have called an emergency meeting of #UNSC to discuss reports of chemical weapons attack in #Syria. Meeting expected on Monday.
— UK at the UN ???? (@UKUN_NewYork) April 8, 2018
The allegations have been denied by the Syrian government and Russia, with the latter separately calling for a Security Council meeting over "threats to international peace and security".
Syrian media accused anti-Assad fighters in Douma of making "chemical attack fabrications", and added that allegations of toxic gas use are an "unconvincing broken record".
Iran has branded the allegations as a "conspiracy" against Syrian President Bashar al Assad, and warned that any US military intervention would "certainly complicate the situation" in Syria and the wider region.
The country's official IRNA news agency said the alleged attack had been condemned by the foreign ministry, which said in a statement: "Such allegations and accusations by the Americans and certain Western countries signal a new conspiracy against the Syrian government and people, and a pretext for military action."
It continued: "With the Syrian army having the upper hand on the ground against the armed terrorists, it would not be rational for it to use chemical weapons."
Mr Trump had earlier tweeted: "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......to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"
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
British Foreign Secretary Boris Johnson described the allegations of a chlorine gas attack in the rebel holdout as "deeply disturbing" and called for an immediate investigation by inspectors from the Organisation for the Prohibition of Chemical Weapons.
The EU has called on Russia and Iran as "supporters of the regime" to use their influence with Damascus to prevent a similar attack.
Last year, the UN was among those blaming Syrian government forces for a deadly sarin gas attack on the opposition-held village of Khan Sheikhun in which at least 100 people died.
That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base, which President Trump said was meant to "prevent and deter the spread and use of deadly weapons".
Responding to the latest allegations, the Russian foreign ministry warned: "The goal of these false speculations, which are not substantiated by any facts, is to cover up terrorists and irreconcilable radical opposition, which opposes political settlement, and to simultaneously try to justify potential external military strikes.
"It is necessary to once again caution that military intervention under false and fabricated pretexts in Syria, where the Russian servicemen stay at the request of the legitimate government, is absolutely unacceptable and may trigger the gravest consequences."