The Syrian Observatory for Human Rights reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating survivors, medics and activists said.
Syria's army denied the government had used any such weapons.
In a statement, President Donald Trump condemned what he called "these heinous actions" by the Bashar al-Assad regime.
"It is clear that this is how Bashar al-Assad operates," the US Secretary of State Rex Tillerson said, "with brutal, unabashed barbarism."
The UK, United Nations and France, among others, also condemned the incident, which would, if confirmed, be one of the deadliest chemical attacks in Syria's civil war.
The spectre of nerve agents in Syria - again
The warplanes are reported to have attacked rebel-held Khan Sheikhoun, about 50km (30 miles) south of the city of Idlib, early on Tuesday, when many people were asleep.
The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees (LCC) network said warplanes had targeted several clinics.US blames Assad over 'chemical attack'
Aftermath of attack in pictures (Warning graphic images)
Why is there a war in Syria?
Pro-government journalists later cited military sources as saying there had been an explosion at an al-Qaeda chemical weapons factory in Khan Sheikhoun that was caused either by an air strike or an accident.
The defence ministry of Russia, President Assad's ally, insisted it had not carried out any air strikes in the vicinity.
The SOHR put the death toll at 58, including 11 children, but Mr Rasoul reported that 67 people had been killed and 300 injured.
The pro-opposition Step news agency said 100 had died.
One aid agency, the Union of Medical Care and Relief Organisations (UOSSM), put the death toll at more than 100.
The EMC said it had stopped counting the victims because there were so many.
The SOHR said it was unable to say what exactly was dropped.
However, the EMC and LCC said it was believed to be the nerve agent Sarin, which is highly toxic and considered 20 times as deadly as cyanide.
Chemical weapons expert Dan Kaszeta said that determining whether Sarin was involved simply by examining video clips is problematic.
He added that Tuesday's attack could have been the result of one of any number of chemical agents as they tend to "behave the same in terms of their physiological effects on the human body".
Sarin is almost impossible to detect because it is a clear, colourless and tasteless liquid that has no odour in its purest form.
Syria's army, in a statement posted on state news agency Sana, said it never had and never would use poison gas.
The government was accused by Western powers of firing rockets filled with Sarin at rebel-held suburbs of Damascus in August 2013, killing hundreds of people.
President Assad denied the charge, blaming rebel fighters, but he did subsequently agree to destroy Syria's chemical arsenal.
Despite that, the Organisation for the Prohibition of Chemical Weapons (OPCW) has continued to document the use of toxic chemicals in attacks in Syria.
In January 2016, it said blood samples taken from the victims of one unspecified attack showed victims had been exposed to Sarin or a Sarin-like substance.
Other reported use of chemical agents:

Photographs and videos posted on social media showed the confused animal apparently trying to get off the road after falling out the back of a lorry.
The tiger had a chain around its neck and appeared to be someone's pet.
The tiger was eventually recaptured, and four men were reportedly spotted bundling it back into the lorry.
One video posted on Facebook showed the animal falling out of the back door of lorry that was driving down the Doha Expressway before running off.
Another video published on Twitter saw the tiger later making its way past cars stuck in heavy traffic.
Footage of the animal finally being recaptured was posted by the al-Watan newspaper.
Twitter users said the incident took place on Tuesday morning, but this could not be confirmed.
It is not clear who owns the tiger, but wealthy individuals in Qatar are believed to keep the animals as pets.
Columnist Sultan Al-Qassemi tweeted: "This obsession of keeping wild animals by rich kids in the Gulf has gone too far."
The White House says it is "confident" Bashar al-Assad's government was behind an apparent chemical attack that killed at least 58 people in north-west Syria.
[NEXT_CONCEPT]
Commuters on a motorway in Qatar's capital, Doha, were astonished to see an escaped tiger on the loose and walking through three lanes of traffic.