Dozens of people have been killed in a suspected chemical attack in rebel-held northern Syria, in one of the largest mass casualty incidents using a toxic gas in the six-year conflict.

The death toll rose to 67 in the hours after the attack on Khan Sheikhun in Idlib province on Tuesday morning. Doctors said the victims exhibited symptoms apparently matching those caused by exposure to deadly sarin gas. Scores more people were injured.

The raids were carried out by planes believed to be loyal to the government of Syrian president Bashar al-Assad. Later, a series of airstrikes on the same town targeted a hospital and two emergency response centres that were recovering and treating victims of the initial strike.

The attack came a day after the US ambassador to the UN, Nikki Haley, said the US government was no longer focused on Assad’s removal from office, and as a two-day conference on Syria’s future, hosted by the EU and UN, began in Brussels.

The international chemical weapons watchdog said it was gathering and analysing information. The French foreign minister, Jean-Marc Ayrault, demanded an emergency UN security council meeting.

Khan Sheikhun houses thousands of refugees from the nearby province of Hama who have fled the fighting there.

“In this most recent attack, dozens of children suffocated to death while they slept,” said Ahmad Tarakji, the head of the Syrian American Medical Society, which supports hospitals in opposition-controlled areas in Syria. “This should strike at the very core of our humanity. How much longer will the world fail to respond to these heinous crimes?”

SAMS said its doctors had determined that the symptoms of the patients were consistent with exposure to organic phosphorus compounds like the nerve agent sarin, which is banned by the chemical weapons convention.

“Everyone is horrified and the children are in total shock,” said Mohammad Hassoun, a spokesman for civil defence rescue workers in the nearby town of Sarmin, which received 14 of the wounded.