At least 70 people have been killed in northern Syria after being exposed to a toxic gas that survivors said was dropped from warplanes, an attack that sparked comparisons to the most infamous act of the country's six-year war.
At least another 100 people were being treated in hospitals in Idlib province where the strike took place at dawn on Tuesday. Several dozen others were transferred to Turkey, some in critical condition.
Condemnation mounted throughout Tuesday as the US, Britain and EU blamed the Syrian government for the carnage, hours before the start of a donor conference on Syria in Brussels.
Donald Trump denounced the carnage as a "heinous" act that "cannot be ignored by the civilised world". But he also laid some of the responsibility on Barack Obama, saying in a statement that the attack was "a consequence of the past administration's weakness and irresolution".
Theresa May said she was appalled by reports of the attack and called for an investigation by the Organisation for the Prohibition of Chemical Weapons. "I'm very clear that there can be no future for Assad in a stable Syria which is representative of all the Syrian people and I call on all the third parties involved to ensure that we have a transition away from Assad. We cannot allow this suffering to continue," she said.
UK-based monitoring group the Syrian Observatory for Human Rights put the latest death toll in Khan Sheikhoun at 72 by Wednesday morning, including 20 children.
The Syrian military said it "categorically denied" responsibility. Russia, which has heavily backed the Syrian regime, said its planes were not operating near Idlib. Early on Wednesday, the Russian defence ministry claimed a Syrian airstrike had hit a "terrorist warehouse" containing an arsenal of "toxic substances" destined for fighters in Iraq. The ministry did not state if the attack was deliberate.
Speaking on BBC Radio 4's Today programme, Hamish de Bretton Gordon, director of Doctors Under Fire and former commanding officer of the UK Chemical, Biological, Radiological and Nuclear (CBRN) Regiment, said this claim was "completely untrue".
"No I think this [claim] is pretty fanciful, no doubt the Russians trying to protect their allies," he said. "Axiomatically, if you blow up sarin, you destroy it."
"It's very clear it's a sarin attack," he added. "The view that it's an al-Qaida or rebel stockpile of sarin that's been blown up in an explosion, I think is completely unsustainable and completely untrue."
Hours after the attack, a hospital treating the injured was also hit. Images taken inside the clinic appeared to depict the blast as it happened. Photographs and videos taken at the scene and in evacuation areas nearby showed rows of small, lifeless children, some with foam visible near their mouths.
Save the Children said at least 11 children were among the casualties.
Jerry Smith, the operations chief of the UN-led team that supervised the removal of Syria's sarin stockpiles following the gas attack on the rebel-held Ghouta area of Damascus four years ago, said: "This absolutely reeks of 2013 all over again." In that attack, more than 1,300 people were killed. The UN said the perpetrators probably had access to the stockpile of sarin held by the Syrian military at the time, as well as the expertise to use it.
In the aftermath of the Ghouta massacre, a UN team supervised the surrender of Syria's sarin supplies, the removal of which was supposed to have been completed early in 2014. However, suspicions have remained that a portion of the stockpile was not declared to inspectors.
Tuesday's attack struck Khan Sheikhun, where there are thousands of refugees from the nearby province of Hama who have fled recent fighting. The town is also on a crossroads between Hama and Idlib and is considered vital to any regime offensive towards the northern city of Idlib.
"In this most recent attack, dozens of children suffocated to death while they slept," said Ahmad Tarakji, the head of the Syrian American Medical Society (Sams), which supports hospitals in opposition-controlled areas in Syria. "This should strike at the very core of our humanity. How much longer will the world fail to respond to these heinous crimes?"
Sams said its doctors had determined that the symptoms of the patients were consistent with exposure to organic phosphorus compounds such as the nerve agent sarin, which is banned by the chemical weapons convention.
Smith said: "If you look at the footage itself, the victims don't have any physical trauma injuries. There is foaming and pinpointed pupils, in particular. This appears to be some kind of organo-phosphate poison. In theory, a nerve agent. What is striking is that it would appear to be more than chlorine. The toxicity of chlorine does not lend itself to the sort of injuries and numbers that we have seen."
Tuesday's strike came days after the US secretary of state, Rex Tillerson, said the Trump administration was no longer prioritising the removal of Assad, and that the Syrian people would ultimately decide his fate.
The US ambassador to the UN, Nikki Haley, made similar comments on Monday, affirming a shift in US policy that began under the Obama administration.
Critics of the stance have said that the absence of a credible threat has given the regime licence to commit war crimes with impunity as its backers, Iran and Russia, steadily claw back years of battlefield losses.
The UN security council said it would hold an emergency meeting on Wednesday to discuss the attack, after a request from Britain and France.
"Everyone is horrified and the children are in total shock," said Mohammad Hassoun, a spokesman for civil defence rescue workers in the nearby town of Sarmin, which received 14 of the wounded from Tuesday's attack. Hassoun said the victims were bleeding from the nose and mouth, had constricted irises and suffered from convulsions.(Source: The Guardian)