The attack in the town of Khan Sheikhun also left dozens suffering respiratory problems and symptoms including vomiting, fainting and foaming at the mouth, the Britain-based Syrian Observatory for Human Rights said.
Hours later, airstrikes hit a hospital in the town where doctors were treating victims of the attack, an AFP correspondent said, bringing down rubble on top of medics as they worked.
The incident brought swift international condemnation, with French Foreign Minister Jean-Marc Ayrault demanding an emergency UN Security Council meeting on the “monstrous” attack.
The EU’s diplomatic chief Federica Mogherini said President Bashar Assad’s government bore “primary responsibility” for the attack, while Syria’s opposition warned it “calls the political process into question,” and demanded a UN probe.
If confirmed, it would be one of the worst chemical attacks since the start of Syria’s civil war six years ago. The Observatory said the attack on a residential area came in the early hours of Tuesday, when a warplane carried out strikes that released “toxic gas.”
It said 11 children were among the dead, with at least 160 injured, and that many people were dying even after arriving at medical facilities.
The monitor could not confirm the nature of the gas, and said the strike was likely carried out by regime warplanes.
Russia’s military denied carrying out any strikes near the town.
The White House described it as a “heinous” chemical attack against civilians by the Syrian regime.
Spokesman Sean Spicer said the attack is “reprehensible and cannot be ignored by the civilized world.”
UN Syria envoy Staffan de Mistura said the “horrific” attack was believed to be chemical and launched from the air, telling reporters in Brussels that there should be a “clear identification of responsibilities and accountability.”
The Syrian opposition’s chief negotiator at peace talks, Mohammed Sabra, said the attack cast new doubt on the UN-led peace process. “If the UN cannot deter the regime from carrying out such crimes, how can it achieve a process that leads to political transition in Syria?” he said.
On Twitter, the head of the opposition High Negotiations Committee Riad Hijab said the “massacre is evidence that it is impossible to negotiate with a regime addicted to criminal behavior.”
Turkish President Recep Tayyip Erdogan told his Russian counterpart Vladimir Putin that “this kind of inhuman attack was unacceptable.” Presidential sources said Erdogan told Putin by phone that the attack threatened peace talks in the Kazakh capital Astana.
German Foreign Minister Sigmar Gabriel said that if the Syrian regime had indeed carried out a chemical attack, “it would be an act of such cruelty as to be without equivalent,” and “another reason we should not deal with the Assad regime in the fight against terrorism.”
French President Francois Hollande accused the Syrian regime of a “massacre.” “Once again the Syrian regime will deny the evidence of its responsibility for this massacre,” Hollande said in a statement.
British Prime Minister Theresa May said she was “appalled” by reports of the attack, adding: “We condemn the use of chemical weapons in all circumstances.”
The UN’s chemical arms watchdog said it was “seriously concerned” by the reports. The Organization for the Prohibition of Chemical Weapons (OPCW) said it was “gathering and analyzing information from all available sources."