The attack was allegedly carried out by Syrian regime jets targeting the town of Khan Sheikhoun, in the south of rebel-held Idlib.
A suspected Syrian regime chemical attack killed scores of 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 Union of Medical Care Organizations, a coalition of international aid agencies that funds hospitals in Syria and which is partly based in Paris, put the death toll as high as 100 with another 400 suffering from respiratory problems.
But the head of the health authority in rebel-held Idlib said more than 50 people had been killed and 300 wounded.
The death toll is likely to rise further, according to the Union of Medical Care Organizations, a coalition of international aid agencies that funds hospitals in Syria and which is partly based in Paris.
The attack was carried out at a village of Khan Sheikhoun to the south of Idlib had initially been hit before strikes on the White Helmets emergency services centre in Khan Sheikhoun and the Al-Rahme hospital.
London-based Syrian Observatory for Human Rights, said the attack was carried out by Syrian regime jets. According to the group, it caused many people to choke, and some had foam coming out of their mouths.
"This morning, at 6:30am, warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine," said Mounzer Khalil, head of Idlib's health authority.
"Most of the hospitals in Idlib province are now overflowing with wounded people," Khalil told a news conference in Idlib.
Warplanes later struck near a medical point where victims of the attack were receiving treatment, the Observatory and civil defence workers said.
Turkish officials said 15 people, mostly women and children, were also brought into Turkey for treatment.
TRT World's Shamim Chowdhury reports from southern Turkish city of Gaziantep.
Regime denies the attack
A Syrian regime military source denied allegations that the regime forces had used chemical weapons.
The United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) previously launched a series of investigations and found that various parties in the Syrian war have used chlorine, sulfur mustard gas and sarin.
An OPCW report published last 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 regime troops for chlorine attacks in Talmenes in March 2014 and Sarmin in March 2015. It also said Daesh had used sulfur mustard gas.
The OPCW had no immediate comment on Tuesday.
However UN said on Tuesday that its war crimes investigators were looking into the alleged chemical weapons attack on a Syrian town in Idlib as well as reports of a subsequent attack on a medical facility where injured people were being treated.
The UN Commission of Inquiry on Syria said that the use of chemical weapons as well as any deliberate targeting of medical facilities "would amount to war crimes and serious violations of human rights law".
"It is imperative for perpetrators of such attacks to be identified and held accountable," said the independent panel led by Brazilian expert Paulo Pinheiro.
Following the 2013 attack, Syria joined the international Chemical Weapons Convention under a U.S.-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.