Three doctors said in interviews that the symptoms they saw were far more serious than they would expect from chlorine, which Syrian Government forces have used as a chemical weapon in the past.
It was not possible to independently verify the reports, but images from the area showed the bodies of at least a dozen men, women and children splayed across the ground between two houses. Video footage showed lifeless bodies wrapped in blankets and packed on the back of a truck. The youngest were wearing nappies.
In another film, several children were seen slumped on hospital beds, apparently unresponsive to the medics and chaos around them.
Syrian Government warplanes in recent months have launched heavy attacks across northern Idlib province, where hundreds of thousands of civilians are squeezed together among what remains of the armed opposition to Syrian President Bashar al-Assad.
Syria's Foreign Ministry denied involvement in the attack, saying it was committed to its obligations under the international Chemical Weapons Convention. It joined in 2013, after launching sarin attacks on several Damascus neighbourhoods - strikes that killed hundreds of civilians and pushed the United States to the brink of military intervention.
"Assad calculates, reasonably, that military dynamics play in his favour. By using chemical weapons and other weapons, he is demonstrating the powerlessness of international actors," said Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies.
The attacks came as European diplomats gathered in Brussels for a flagship conference aimed at pledging billions of dollars for Syria's reconstruction, six years into a war that has shattered much of the country and prompted refugees to pour out across the Middle East and Europe.
What began as a peaceful movement burgeoned into a war between Assad and armed opposition groups that has since splintered into a host of linked but separate conflicts. Rebel forces are nearing defeat by a coalition of Syrian, Russian and Iran-backed forces. Isis (Islamic State) militants are also under pressure by an array of troops, with the White House predicting that the battle for the extremist group's most important stronghold, Raqqa, may begin before the northern summer.
Against this chaotic backdrop, European nations are banking on the possibility that they can use funding as leverage to influence any future transition, which most now accept will allow Assad to remain in power in the short-term.
"Rightly or wrongly, Assad thinks that his narrative has prevailed in Western countries and that they will over time shift their policies and rehabilitate him," said Hokayem.
Doctors and activists in rebel-held areas have accused the government sharply increasing chemical attacks across Idlib, Aleppo and Hama provinces since the end of last year. A list of the dead compiled by local activists today included 70 names.
In the Sheikh Khanoun attack, Samer al-Youssef, a local resident, described watching people run toward the homes of their extended families, wrenching open the doors to find them dead inside.
"We did our best, but we couldn't save people. Around 30 percent of those who were brought to us were dead on arrival," said Usama Darwish, a local doctor.
Although a nationwide ceasefire has technically been in place across Syria since late December, civilians and rebel groups now say it exists in name only.
"People are terrified. They don't know where to go," said Ahmad Rahhal, a 22-year-old activist. "They can't cross into Turkey because the borders are closed, but if they stay in their houses, they will be attacked by bombs. What can they do?"
As a displacement crisis burgeons on its southern border, Turkey has limited new Syrian arrivals to those seeking medical treatment, often in the wake of attacks. Reports suggested that ambulances were lined up at the border crossing, ready to bring the next wave of casualties to Turkish hospitals.
