Foreign Secretary Boris Johnson has branded Russia a "super villain" in Syria as he told Moscow it still has time to be on the "right side of the argument" in the conflict.
Mr Johnson, who was widely attacked for failing to get the G7 to back his bid for new sanctions against senior Russian and Syrian figures in the aftermath of the chemical weapons attack on civilians in Idlib province, continued to criticise Vladimir Putin's stance.
The Foreign Secretary called on Russia to change course in Syria, asking if the country wanted to be part of a "league of super villains".
"We need to show the Russians the horrific nature of the regime they are backing in terms they cannot fail to understand. This is in fact an opportunity for Russia.
"Moscow has reached the high point of its influence in Syria. They still have innumerable rebel groups to subdue, and they find themselves in a league of super villains with Hizbollah and Assad. Is that what they want?" he wrote in the Sunday Telegraph.
Mr Johnson also branded Syrian president Bashar Assad an "arch terrorist".
"Assad uses chemical weapons because they are not only horrible and indiscriminate. They are also terrifying.
"In that sense he is himself an arch-terrorist, who has caused such an unquenchable thirst for revenge that he can never hope to govern his population again.
"He is literally and metaphorically toxic, and it is time Russia awoke to that fact. They still have time to be on the right side of the argument."
The Foreign Secretary insisted it was "highly likely" Assad was responsible for the chemical weapons atrocity as Mr Johnson insisted America "could of course strike again".
"British scientists have analysed samples from the victims of the attack. These have tested positive for Sarin or a Sarin-like substance.
"The UK, the US and all our key allies are of one mind: we believe that this was highly likely to be an attack by Assad, on his own people, using poison gas weapons that were banned almost 100 years ago, under the 1925 Geneva protocol."
Mr Johnson said the chemical assault had changed the West's stance on Syria.
"Let us face the truth: Assad has been clinging on. With the help of Russians and Iranians, and by dint of unrelenting savagery, he has not only recaptured Aleppo. He has won back most of "operational" Syria.
"Before the chemical weapons attack on April 4, the West was on the verge of a grim consensus - that it would be more sensible to concentrate on the fight against the terrorists of Daesh, and to accept reluctantly that removing Assad - though ultimately essential - should await a drawn out political solution."