On Wednesday Damascus ally Russian Federation vetoed a U.N. Security Council resolution to condemn the attack and push the Syrian government to cooperate with investigators. This is another step in the investigation of the chemical weapons attack in which more than 100 people were murdered and dozens more were injured. Neither did Russian Federation deny the event occurred: Moscow claimed the deaths were the result of a regime airstrike on a chemical weapons stockpile on the ground, controlled by "terrorists".
Images of the aftermath, showing victims convulsing and foaming at the mouth, sent shockwaves around the world. Sufferers were taken to hospitals across the border in Turkey.
USA officials were the first to say they believed Sarin gas was used in the attack, but they offered no concrete evidence.
But the attack would not be the first time the Syrian government has used chemical weapons against its own citizens.
In 2013, Syria was blamed for a chemical attack that hit the rebel-held Damascus suburb of Ghouta, in which activists say 1,400 people died.
As BBC pointed out, the situation in Syria remains highly volatile, with Russian and Iran, another one of Assad's few military backers, threatening retaliation if the USA launches more air strikes against the Syrian government.
Mr Rycroft said: "The United Kingdom therefore shares the USA assessment that it is highly likely that the regime was responsible for a sarin attack on Khan Sheikhoun on April 4".
"This story is not convincing by any means". Washington said that account was not credible, and rebels have denied it.
Trump's administration initially took a hands-off approach to Syria, with Assad raising the possibility the new USA president could even be a "natural ally".
Then there is the detailed narrative, provided by the Americans who tracked the aircraft they say launched the attack, from its base, to the target location, and then home again.
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Denouncing the attack as a "heinous" act, he highlighted the need to provide peace and comfort to the victims, especially to those whose loved ones died in the incident. Their timing and location have been verified by independent researchers.
It is "highly likely" the Assad regime is responsible, said United Kingdom ambassador to the UN Matthew Rycroft.
UK Prime Minister Theresa May said on Thursday it was "highly likely" the Assad regime was behind the attack.
Western powers blame the gas attack, which killed scores of civilians - many of them children - on Assad's forces.
Turkey, which treated numerous wounded, says it has "concrete evidence" Sarin was used.
Assad said he would only allow an "impartial" external investigation involving "unbiased countries", according to AFP.
But he said his forces had not been diminished by the United States strike. The SDF say their goal is to ultimately recapture the city, which has served as the de facto capital the so-called Islamic State.
A spokesman for the USA military coalition that operates bombing missions against the Islamic State in Syria denied the report.
US President Donald Trump retaliated last Friday with a "warning shot" barrage of 59 Tomahawk missiles which struck the regime-operated Shayrat airbase near Homs - the first direct action against Mr Assad's forces taken by the US in more than six years of the civil war.