The attack killed at least 58 people, including children, in the northwestern province of Idlib on Tuesday, a monitoring group, medics and rescue workers in the rebel-held area said.
The British-based Syrian Observatory for Human Rights said the attack was believed to have been carried out by Syrian government jets. It caused many people to choke, and some to foam at the mouth. But the Syrian military denied responsibility and said it would never use chemical weapons.
The head of the health authority in rebel-held Idlib said more than 50 people had been killed and 300 wounded. The Union of Medical Care Organizations, a coalition of international aid agencies that funds hospitals in Syria, said at least 100 people had died.
The White House condemned what it called a "reprehensible" and "intolerable" chemical attack in Syria Tuesday and pinned the blame squarely on Assad's regime.
White House spokesman Sean Spicer said an "extremely alarmed" President Donald Trump had been briefed extensively on the attack, and suggested it was in the "best interest" of the Syrians for Assad not to lead the country.
"Today's chemical attack in Syria against innocent people, including women and children, is reprehensible," Spicer said, adding that the administration was "confident" in its assessment that Assad was to blame.
Top EU diplomat Federica Mogherini said Assad held “primary responsibility” in the suspected gas attack. “Today the news is awful. This is a dramatic reminder that the situation on the ground still continues to be dramatic in many parts of Syria,” she told reporters.
UN spokesman Stéphane Dujarric said: “Any sort of report of use of chemical weapons, especially on civilians is extremely alarming and disturbing. Any use of chemical weapons anywhere constitutes a threat to peace and security and is a serious violation of international law.”
But the Assad regime said it was not to blame for the catastrophe.
“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 Syrian army command said in a statement.
The Russian Defence Ministry said its aircraft had not carried out the attack as the UN Security Council prepared to meet on Wednesday to discuss the incident. Assad has enjoyed staunch military backing from Iran and Russia in the war.
Activists in northern Syria circulated pictures on social media showing a man with foam around his mouth, and rescue workers hosing down almost-naked children squirming on the floor.
Mounzer Khalil, head of Idlib’s health authority, said hospitals in the province were overflowing with victims.
“This morning, at 6:30am, warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine,” he told a news conference.
Warplanes later struck near a medical point where victims of the attack were receiving treatment, the Observatory and civil defence workers said.
The civil defence, also known as the White Helmets - a rescue service that operates in opposition areas - said jets struck one of its centres in the area and the nearby medical point.
The incident reported at Khan Sheikhoun would be the deadliest chemical attack in Syria since sarin gas killed hundreds of civilians in Ghouta near Damascus in August 2013. Western states said the Syrian government was responsible for that attack. Damascus blamed rebels.
Idlib’s population has ballooned, with thousands of fighters and civilians shuttled out of Aleppo city and areas around Damascus that the government has retaken in recent months as Assad has gained the upper hand in the war.
The United States has also launched a spate of air strikes in Idlib this year, targeting jihadist insurgents.
Following the 2013 attack, Syria joined the international Chemical Weapons Convention under a US-Russian deal, averting the threat of US-led military intervention.
Under the deal, Syria agreed to give up its toxic arsenal and surrendered 1,300 tonnes of toxic weapons and industrial chemicals to the international community for destruction.
UN investigators found, however, that it continued to use chlorine, which is widely available and difficult to trace, in so-called barrel bombs, dropped from helicopters.
Although chlorine is not a banned substance, the use of any chemical is banned under 1997 Chemical Weapons Convention, to which Syria is a member.
Damascus has repeatedly denied using such weapons during the six-year war, which has killed hundreds of thousands and created the world’s worst refugee crisis.
(FRANCE 24 with AFP, REUTERS)