Medical relief organization Syrian American Medical Society (SAMS) and the civil defense service said on Sunday that 49 people had died in the attack late on Saturday in the town of Douma.
"It is a matter of grave concern that chemical weapons continue to be used, especially on civilians," the EU said in a statement. "The European Union condemns in the strongest terms the use of chemical weapons and calls for an immediate response by the international community."
US President Donald Trump accused Russia and Iran over the alleged attacks: "Russia and Iran are responsible for backing Animal Assad. Big price to pay," he said via Twitter.
Britain's Foreign Secretary Boris Johnson said on Sunday the reports were "deeply disturbing," and warned that Russia must not try to block an international investigation. Johnson claimed the Syrian forces had used poison gas in at least four attacks since 2014.
The UK mission to the United Nations said on Sunday the Security Council was likely to meet on Monday afternoon over the attack, at the request of nine members: "UK, France, US, Poland, Netherlands, Sweden, Kuwait, Peru and Cote d’Ivoire have called an emergency meeting of #UNSC to discuss reports of chemical weapons attack in #Syria. Meeting expected on Monday."
France's foreign minister, Jean-Yves Le Drian, called for the UN Security Council to meet quickly to examine the situation. He added that Paris would assume all its responsibilities in the fight against chemical weapons.
President Emmanuel Macron said he "strongly condemned the chemical attacks" against civilians in Douma during a call US President Donald Trump on Sunday. Both leaders were said to have agreed to coordinate "a strong, joint response" to what they considered a "horrific" attack and concluded that the Syrian regime "must be held accountable for its continued human rights abuses."
Turkey also condemned the attack. A statement from Turkey's foreign ministry said there was a "strong suspicion" the Assad regime was responsible for it. "We strongly condemn the attack and we have the strong suspicion it was carried out by the regime, whose record on the use of chemical weapons is known by the international community," the ministry in Ankara said.
Pope Francis on Sunday joined the international censure: "Nothing, nothing can justify the use of such devices of extermination against defenseless people and populations," the pope told thousands of people gathered in St Peter's Square.
Saudi Arabia called on the international community to "shoulder its responsibility toward protecting civilians in Syria," according to Turkey's Anadolu Agency which cited the Saudi SPA news agency.
Syria's allies react
Russia warned the US against carrying out a "military intervention on fabricated pretexts" in Syria. The foreign ministry said that "a military intervention under far-fetched and fabricated pretexts in Syria, where there are Russian soldiers at the request of the legitimate Syrian government, is absolutely unacceptable and could have the most dire consequences."
Iran's foreign ministry said the reports were an excuse for military action against Damascus, the IRNA news agency reported. Spokesman Bahram Ghasemi was quoted as saying: "Such claims and accusations by the Americans and some western countries are an indication of a new plot and excuse for military action against the Syrian government and nation."
Syria denies ever using chemical weapons during the seven-year war, and says it eliminated its chemical arsenal under a 2013 agreement brokered by the US and Russia.