Many of the victims were seen to be choking and fainting following the attack, and some had foam coming out of their mouths, according to medical sources who described the symptoms as related to a gas attack, as cited by media reports.
Pictures circulated by activists showed members of the volunteer White Helmets rescue group using hoses to wash down the injured, as well as two men with white foam around their mouths.
The pro-opposition Edlib Media Centre (EMC) also posted photos of people receiving treatment online, with some images showing what appeared to be the bodies of at least seven children in the back of a pick-up truck, France 24 reported.
Later a rocket slammed into a hospital treating the victims of the alleged gas attack, according to an AFP correspondent.
The Syrian National Coalition, an alliance of opposition groups, has accused President Bashar Assad's government of carrying out the gas attack and demanded a UN investigation.
The National Coalition demands the Security Council convene an emergency session...open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable," the body said in a statement, as quoted by France 24.
The UN Commission of Inquiry on Syria said it is looking into the alleged gas attack, as well as the hospital attack, stressing that the use of chemical weapons and any deliberate targeting of medical facilities "would amount to war crimes and serious violations of human rights law."
"It is imperative for perpetrators of such attacks to be identified and held accountable," the commission said, as quoted by Reuters.
The Syrian army has denied allegations that it was behind the attack, despite allegations from the Syrian Observatory for Human Rights and other pro-opposition groups.
"We deny completely the use of any chemical or toxic material in Khan Sheikhoun town today and the army has not used nor will use in any place or time, neither in past or in future," the army said in a statement, as quoted by Reuters.
Earlier, a Syrian military source told Reuters that Damascus had not used chemical weapons, and dismissed the accusations as rebel propaganda.
The army "has not and does not use them, not in the past and not in the future, because it does not have them in the first place," the source said.
AFP also reported that a Syrian security source called the gas attack report a "false accusation."
“Based on the reports it might be that the Syrian Army targeted a place that was used as a chemical weapons factory or that stocked chemicals by the rebels,” Jamal Wakeem, a professor of history and international relations at the Lebanese University in Beirut, told RT.
Wakeem said he doesn’t believe the Syrian Army would be so “desperate” as to use chemical weapons, given the fact that it has been “achieving major success on various fronts.”
He went on to state that although rebels have previously accused the Syrian government and Russian military of using chemical weapons, that news “turned out to be false,” noting that there was instead evidence that rebels had used chemical weapons in Syria and Iraq.
The Russian military said in a statement that it did not conduct any airstrikes in the Idlib Governorate.