Watching the slow expulsion of the frightful Islamic State from its last strongholds in Syria, it was almost possible for forget that events have turned in favour of the survival of a regime equally as frightful for its opponents.

It was even possible to hope that since Bashar Al-Assad owed his survival to Russia's intervention, better counsels might prevail in Syria's future.

That hope has been dashed by the chemical weapon that killed and injured men, women and children in the town of Khan Sheikhoun yesterday.

The Assad regime denies responsibility but, as the world well remembers, it is not the first time it has employed deadly sarin gas against Syrian civilians.

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The first time was in direct defiance of a "red line" warning issued by the United States' former President Barrack Obama.

He failed to back it up militarily as he had threatened, settling for a deal with Russia under which Assad agreed to hand over his chemical arsenal for destruction and abide by the International Chemical Weapons Convention.

There is not much doubt Assad has done it again. The head of the health authority in the rebel-held area has reported that aircraft attacked the town at 6.30am with gases believed to be sarin and chlorine.

The warplanes struck again at a medical centre where victims were seeking treatment. Shortly thereafter Reuters photographed people wearing oxygen masks and protective clothing carrying dead bodies.

Corpses were lying on the ground. Pictures posed on social media showed people suffering the effects of the gas and rescue worker desperately hosing down near naked children.

What is the world to do? The UN Security Council is to discuss the war crime today but it is Russian President Vladimir Putin who holds all the cards in Syria now.

Having saved Assad from likely defeat last year Putin has made Assad reliant on Russia's continued backing.

Meanwhile the US under Donald Trump has dropped its opposition to Assad being part of Syria's future governing arrangements.

Trump's only interest appears to be the defeat of Isis and beyond that, he has seemed content for Syria's fate to be resolved in talks between Russia, Turkey and Iran.

Of the three, only Turkey would like to see the back of Assad but it is more concerned to ensure his Kurdish opponents do not establish their own state on Turkey's border.

Assad's survival has looked so assured for the past year that it makes no sense for his regime to commit an atrocity like this.

It can only remind the world why so much of Syria rose up against him in the "Arab Spring" and how he ended that time of hope by his willingness to bomb Syrian cities.

His defiance of Obama's "red line" against chemical weapons enabled Putin to seize the initiative in Syria but this second chemical attack puts Putin in the spotlight.

Putin was a co-sponsor of the 2013 deal to destroy Assad's chemical weapons, he can not ignore this attack.

Putin might not care for the victims but he cares very much for his reputation for strength and his ambition to make Russia respected in the world.

Unless he is willing to be tarred by the same brush he needs to ditch Assad now.