In August 2013, more than 1,000 people were killed in the Damascus area by the banned toxin sarin, leading President Bashar Assad to agree to eliminate his country's chemical weapons program, which he had previously denied having.
The incident brought swift global condemnation, with France and Britain demanding an emergency UN Security Council meeting and President Francois Hollande denouncing a "massacre".
After the initial strikes early Tuesday, a hospital in Khan Shaykhun was also bombarded as it treated victims, according to Agence France-Presse.
Airstrikes launched by the Syrian government over the city of Khan Sheikhoun released a poisonous gas that choked civilians, according to the BBC.
Israel's prime minister is calling for the world to rid Syria of chemical weapons after a suspected chemical attack killed dozens of people there.
"I call on the worldwide community to fulfill its obligation from 2013 to fully and finally remove these disgusting weapons from Syria", Netanyahu said during the annual state memorial ceremony for President Chaim Herzog.
Tarik Jasarevic, spokesman for the World Health Organization in Geneva, said in an emailed statement that the agency is contacting health providers from Idlib to get more information about Tuesday's incident.
Syria's government officially joined the Chemical Weapons Convention and turned over its chemical arsenal in 2013, as part of a deal to avert USA military action.
The opposition accused the Syrian air force of being behind the attack.
Dozens were killed in a suspected chemical attack in a town in Syria's rebel-held northern Idlib province on April 4.
The attack, which has been blamed on the Syrian regime by activists and condemned by global leaders, has reportedly left hundreds injured.
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The Syrian military denied using chemical weapons and said rebels were responsible.
The Iranian Resistance's President-elect Maryam Rajavi described as "a major war crime" the chemical attack on Idlib and the ruthless carnage of defenseless people, particularly the innocent children.
"If this is shown to be the work of the regime, it is further evidence of the atrocities perpetrated against the Syrian people over six years of appalling conflict", Johnson said.
The Syrian activists claimed the attack was caused by an airstrike carried out either by Syrian government or Russian warplanes.
A Turkey-based Syrian woman whose niece, husband and one-year-old daughter were among those killed said the warplanes struck early, as residents were still in their beds.
The attack had killed more than 50 people and wounded 300, he said.
A series of investigations by the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) found that various parties in the Syrian war had used chlorine, sulphur mustard gas and sarin.
Washington backed its allies but blamed the Obama administration which it said "did nothing" - despite having warned President Bashar al- Assad's regime in 2012 that use of chemical weapons would constitute the crossing of a "red line".
Under the deal, Syria agreed to give up its toxic arsenal and surrendered 1,300 tonnes of toxic weapons and industrial chemicals to the global community for destruction. Diplomats said that meeting could be moved up to later on Tuesday.
The doctor, who can not be named for security reasons, said tests were needed to identify if sarin gas was to blame.
"We obviously are very concerned about what happened with the Syrian chemical weapon attack so we'll be having an emergency meeting tomorrow morning in the open chamber", U.S. Ambassador to the United Nations Nikki Haley told reporters.