The death toll from a suspected chemical weapons attack on a rebel-held Syrian town has risen to 72, 20 of them children, a monitoring group has said.
"There were also 17 women among the dead and the death toll could rise further because there are people missing," the Syrian Observatory for Human Rights said.
On Wednesday, EU president Donald Tusk declared that Bashar al-Assad's regime was chiefly responsible for the attack.
"The Syrian regime (has) the primary responsibility for the atrocities," Tusk said. "But all those who support the regime share the moral and political responsibility.
"The indiscriminate killing of children, women and men with chemical weapons is another tragic reminder of the brutality of this conflict, and the Syrian regime."
But Moscow, which holds a veto, defended its Damascus ally saying that while Syrian aircraft had carried out a strike, the chemicals were part of a "terrorist" stockpile of "toxic substances" that had been hit on the ground.
"According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhun," the Russian defence ministry said in a statement.
Earlier on Wednesday, spokeswoman for UN Secretary General António Guterres, Stéphane Dujarric, said the UN was still seeking clarification on details of the attack.
"We are not currently in a position to verify the reports.
"I saw that earlier today the OPCW fact finding mission has already announced that it has begun gathering information to attempt to confirm that chemical weapons were used.
"Obviously, 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 international peace and security and is a serious violation of international law," she said.
Rebel groups led by former Al-Qaeda affiliate Fateh al-Sham Front vowed revenge for Tuesday's strike in the town of Khan Sheikhun in Idlib province in the northwest.
The Syrian military earlier denied responsibility and said it would never use chemical weapons.
International NGO Save the Children says medics in the area told them nearly a third of the casualties they have seen are children.
"They were struggling to breathe and barely conscious, with running noses and contracted pupils – doctors say these symptoms are consistent with the use of nerve agents such as Sarin," said the group's Syria Director Sonia Khush.
"If a banned chemical substance is confirmed, this would be in clear violation of international law and a worrying indication that not all chemical weapons have been destroyed in Syria."
Watch: UN Security Council to meet over the attack
Australia deplores the attack
Australia's political leaders have widely condemned the chemical weapons attack on an opposition-held town in Syria that has killed scores of people.
Prime Minister Malcolm Turnbull has called the attack a shocking war crime, while Labor leader Bill Shorten and foreign affairs spokeswoman Penny Wong say the barbaric and criminal act highlights the brutality of the Assad regime towards its own people.
Greens leader Richard Di Natale has called it appalling and says the attacks must be internationally condemned without reservation.
Britain, France, US present UN draft resolution condemning Syria chemical attack
Britain, France and the United States on Tuesday presented a draft resolution to the UN Security Council condemning the suspected chemical attack in Syria and demanding a full investigation as soon as possible.
The text, obtained by AFP, called on the Organisation for the Prohibition of Chemical Weapons (OPCW) to report quickly on its findings on the attack earlier in the day on the rebel-held town of Khan Sheikhun in Idlib province that killed at least 58 civilians.
Warplanes later struck near a medical point where victims of the attack were receiving treatment, the British-based Syrian Observatory for Human Rights and civil defense workers said.
The civil defense, also known as the White Helmets - a rescue service that operates in opposition areas - said jets struck one of its centers in the area and the nearby medical point.
A total death toll is still unclear.
The Syrian Observatory for Human Rights said the attack killed at least 58 people and was believed to have been carried out by Syrian government jets. It caused many people to choke and some to foam at the mouth.
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.
Watch: 'Chemical weapons are a threat to international peace
Alleged image from hospital attack:
US Secretary of State Rex Tillerson has blamed the Assad government and their backers, Russia and Iran.
"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism," he said in a statement.
"Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions."
Syrian Observatory for Human Rights Director Rami Abdulrahman told Reuters the assessment that Syrian government warplanes were to blame was based on several factors such as the type of aircraft, including Sukhoi 22 jets, that carried out the raid.
The Syrian army has denied responsibility for any chemical attack.
"The army has not used nor will use in any place or time neither in past or in future," Syrian army command said in a statement.
Watch: Chemical attack in Idlib came from the air
The Russian Defence Ministry said its aircraft had not carried out the attack. The UN Security Council is expected to meet on Wednesday to discuss the incident.
Reuters photographs showed people breathing through oxygen masks and wearing protection suits, while others carried the bodies of dead children, and corpses wrapped in blankets were lined up on the ground.
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.
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.
Mounzer Khalil, head of Idlib's health authority, said hospitals in the province were overflowing with victims.
"This morning, at 6:30 a.m., warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine," he told a news conference.
The White House called the attack an "intolerable act" and said President Donald Trump was alarmed by the reports.
French President Francois Hollande directly blamed Syrian government forces and said President Bashar al-Assad's allies were emboldening him to act with impunity.
Assad has enjoyed staunch military backing from Iran and Russia in the war.
Britain said he would be guilty of a war crime if it were proved that his regime was responsible. British Prime Minister Theresa May called for an investigation into the attack.
The United Nations envoy for Syria said the "horrific" chemical attack had come from the air. The UN Security Council is expected to be briefed on the attack on Wednesday.
In February, Russia, backed by China, cast its seventh veto to protect Assad's government from council action, blocking a bid by Western powers to impose sanctions over accusations of chemical weapons attacks during the conflict.
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, sulfur mustard gas and sarin.
A joint UN-OPCW report published in October said government forces used chlorine in a toxic gas attack in Qmenas in Idlib province in March 2015. An earlier report by the same team blamed Syrian government troops for chlorine attacks in Talmenes in March 2014 and Sarmin in March 2015. It also said Islamic State had used sulfur mustard gas.
The OPCW said it had begun "gathering and analyzing information from all available sources" about the suspected Khan Sheikhoun attack.
Turkey, which backs the anti-Assad opposition, said the attack could derail Russian-backed diplomatic efforts to shore up a ceasefire.
Turkey's state-run Anadolu news agency said 15 people hurt in the attack, mostly women and children, had been taken to Turkey.
Footage from Turkey's Dogan news agency showed at least four people being brought out of ambulances on stretchers in the Turkish border town of Reyhanli by medical staff wearing face masks. One was a young boy.
An official at the Turkish Health Ministry said Turkey's disaster management agency was first "scanning those arriving for chemical weapons, then decontaminating them from chemicals" before they could be taken to hospital.
Idlib province contains the largest populated area controlled by anti-Assad rebels - both nationalist Free Syrian Army groups and powerful Islamist factions including the former al Qaeda-linked Nusra Front.
Turkish President Tayyip Erdogan and Russian President Vladimir Putin discussed the suspected attack, Turkish presidential sources said. They said the two leaders had also emphasized the importance of maintaining the much-violated Syrian ceasefire.
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-OPCW 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.