British Foreign Secretary Boris Johnson Petros Giannakouris / AP

British Foreign Secretary Boris Johnson has called for fresh sanctions to be brought against Russia in response to a chemical weapons attack on Syrian civilians.

Johnson, who last week cancelled a visit to Moscow planned for April 10, claimed that the threat of new economic sanctions would force the Kremlin to reconsider backing Syrian President Bashar Assad.

“We cannot miss this moment. It is time for Putin to face the truth about the tyrant he is still propping up,” Johnson told British tabloid The Sun. “He must now understand that Assad is now toxic in every sense. He is poisoning the innocent people of Syria with weapons that were banned 100 years ago. And he is poisoning the reputation of Russia.”

Johnson is expected to set out new measures at a G7 meeting in Italy on Monday.

More than 70 people died in the Syrian province of Ildib on Wednesday after being hit by a cloud of deadly sarin gas.

Syrian President Bashar Assad has been widely blamed for the attack, with eyewitnesses describing how military jets dropped canisters of the gas across the region. The reports have led Russia to face widespread condemnation for supporting Assad and failing to ensure that the regime had destroyed its stockpile of chemical weapons.

The Kremlin has been among those to dismiss the theory, instead saying that the gas had spread from a Syrian warehouse containing “toxic substances” after it had been hit by a terrorist attack.

The attack spurred U.S. President Donald Trump to launch a missile strike on a government-held airbase close to the city of Homs.

Johnson urged Western politicians to take advantage of Trump's intervention.

“There is no doubt that this U.S. action is a game changer in Syria. We need to make it clear to Putin that the time to back Assad has gone. By continuing to support a man who gasses his own people he is damaging Russia further in the eyes of the world,” Johnson said.

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Britain to Propose New Russia Sanctions After Syria Gas Attack