U.K. Foreign Secretary Boris Johnson has called Syrian President Bashar al-Assad an "arch-terrorist" and said Russia still has "time to be on the right side of the argument."
In an interview published on April 16 in The Sunday Telegraph newspaper, Johnson also warned that the United States could attack again after the April 7 cruise-missile strike made in retaliation for Assad's reported use of chemical weapons on his own people.
Johnson vowed that Britain and its allies would gather evidence for "war crimes prosecutions for those responsible" for the chemical attack.
Russia, along with Iran, is Assad's key supporter in the six-year Syrian civil war against rebels supported by the United States and Turkey.
Johnson recently canceled a visit to Russia over the alleged chemical attack and has pushed for increased sanctions against the Syrian and Russian militaries.
Syria has denied it carried out a chemical-weapons attack, but U.S. and other Western leaders have said there was no doubt Assad's government was responsible for the attack that killed at least 70 people.
Johnson said Assad used chemical weapons because they are "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 told the Telegraph.
"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," he added.
He said the Russians could join a coalition of more than 60 countries in the fight against Islamic State (IS) militants, which would allow them to maintain their strategic interests in Syria.