Foreign secretary Boris Johnson has offered the Russians an opportunity to end the Syria crisis, saying they can still come out on the "right side of the argument".
Writing for the Sunday Telegraph, Johnson said the UK and its allies will gather evidence to prosecute those who were responsible for a recent chemical weapons attack in Syria.
US president Donald Trump launched airstrikes against Syria after a gas attack in the country that killed 70 people.
However, Johnson has faced criticism for not persuading G7 countries to agree to sanctions against Russia for its ties with Assad.
Now, Johnson has said: "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."