British Foreign Secretary Boris Johnson said on Sunday that Syrian President Bashar al-Assad is an "arch-terrorist" and it is time Russia realised he is "literally and metaphorically toxic".

Johnson said in a Sunday Telegraph newspaper article that Assad's ally Moscow still has time to be on the "right side of the argument".

"Assad uses chemical weapons because they are not only horrible and indiscriminate. They are also terrifying," Johnson wrote.

"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."

'Whole families dead in their beds': Syrian gas attack kills scores Read More »

Johnson was widely criticised for failing to get the G7 group of leading industrialised nations to back his bid for new sanctions against senior Russian and Syrian figures following an alleged chemical weapons attack in Syria's Idlib province on 4 April that killed at least 87 civilians and caused an international outcry.

Syria has denied responsibility and Russia has claimed that a rebel weapons cache was hit, an impossibility according to chemical weapons expert Hamish de Bretton-Gordon, writing for MEE.

Johnson said the incident had changed the West's stance on Syria.

"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," he wrote.

The US hit the Syrian airbase from which it said the assault was launched with cruise missiles, days after the attack.

"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 alleged chemical attack, the West was "on the verge of a grim consensus," which had now changed, said Johnson.

The consensus had been that it would be more sensible to concentrate on the fight against the Islamic State group and to accept reluctantly that removing Assad, "though ultimately essential - should await a drawn out political solution".