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, air strikes 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 al-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 part of Khan Sheikhun came in the early hours of Tuesday morning, 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 government warplanes.
Russia's military, which has been fighting in support of Assad's government since September 2015, denied carrying out any strikes near the town.
- 'Convulsions, pinpoint pupils' -
An AFP journalist in Khan Sheikhun saw a young girl, a woman and two elderly people dead at a hospital, with foam still visible around their mouths.
Doctors at the facility were using basic equipment, some not even wearing lab coats, and attempting to revive patients who were not breathing.
A father carried his dead little girl, her lips blueish and her dark curls visible, wrapped in a blue sheet.
As doctors worked, a warplane circled overhead, striking first near the facility and then hitting it twice, bringing rubble down on medics and patients.
It was not immediately clear how many people may have been injured or killed in the strikes.
In a video posted online by Idlib's local medical directorate, a doctor described patient symptoms as he treated a child.
"We are seeing unconsciousness, convulsions, pinpoint pupils, severe foaming, and lack of oxygen," he said.
Khan Sheikhun is in Syria's Idlib province, which is largely controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front.
The province is regularly targeted in strikes by the regime, as well as Russian warplanes, and has also been hit by the US-led coalition fighting the Islamic State group, usually targeting jihadists.
Syria's leading opposition group, the National Coalition, blamed Assad's government for the attack and demanded the UN "open an immediate investigation" and hold those responsible to account.
"Failure to do so will be understood as a message of blessing to the regime for its actions," it said.
Syria's government officially joined the Chemical Weapons Convention and turned over its chemical arsenal in 2013, as part of a deal to avert US military action.
That agreement came after hundreds of people -- up to 1,429 according to a US intelligence report -- were killed in chemical weapons strikes allegedly carried out by Syrian troops east and southwest of Damascus.
But there have been repeated allegations of chemical weapons use by the government since then, with a UN-led investigation pointing the finger at the regime for at least three chlorine attacks in 2014 and 2015.
The government denies using chemical weapons and has accused rebels of using banned weapons.
- 'Inhuman attack': Erdogan -
More than 320,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.
Successive rounds of peace talks, including a UN-sponsored meeting in Geneva last week, have failed to produce a political breakthrough.
And the attack cast new doubt on the peace process, said the Syrian opposition's chief negotiator Mohamad Sabra.
"If the United Nations cannot deter the regime from carrying out such crimes, how can it achieve a process that leads to political transition in Syria?" he told AFP.
Rebel backer Turkey has worked in recent months with regime ally Russia to advance negotiations, but Turkey's president Recep Tayyip Erdogan also warned Tuesday the attack could endanger talks.
In a phone call with his Russian counterpart Vladimir Putin, he said the "inhuman attack... risked wasting all the efforts" made on negotiations, presidential sources in Turkey said.
Donor nations are meeting Tuesday in Brussels to discuss Syria's future, amid confusion over Washington's position on Assad's fate.
Last month, President Donald Trump's administration appeared to depart from previous policy by suggesting Assad's departure was not a priority.
But on Monday, US ambassador to the United Nations Nikki Haley called Assad a "war criminal" and "a hindrance to peace" and said the US would not accept him standing for reelection.
