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More than 50 people, including children, have been killed, and hundreds wounded in a suspected Syrian government chemical attack.
Photographs of the attack in the rebel-held province of Idlib show people breathing through oxygen masks and wearing protection suits, while others carried the bodies of dead children, and corpses wrapped in blankets lined the ground.
Mounzer Khalil, head of Idlib's health authority, said hospitals in the province were overflowing with victims, with more than 50 people killed, and at least 300 wounded.
The Union of Medical Care Organizations, a coalition of international aid agencies, said at least 100 people had died, and the Syrian Observatory for Human Rights believed the attack was carried out by Syrian jets.
The observatory and civil defence workers said warplanes struck near a medical point where victims of the attack were receiving treatment.
The UN envoy for Syria said the "horrific" chemical attack had come from the air.
The 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.
However, the Syrian military has denied responsibility for Wednesday's (local time) attack and said it would never use chemical weapons.
The Russian Defence Ministry also said its aircraft had not carried out the attack.
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.
Britain said he would be guilty of a war crime if it were proved his regime was responsible.
Turkey, which backs the anti-Assad opposition, said the attack could derail Russian-backed diplomatic efforts to shore up a ceasefire.
Assad has enjoyed staunch military backing from Iran and Russia in the war.
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, but Damascus blamed rebels.
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 1300 tonnes of toxic weapons and industrial chemicals; however an investigation found that it continued to use chlorine, in the form of barrel bombs.
The UN Security Council is expected to meet on Thursday to discuss the incident.