The United States has declared that the slaughter of Christians, Yazidis and Shiites by Islamic State of Iraq and the Levant amounts to genocide.
It came as Vladimir Putin said Russia was capable of scaling up its military presence in Syria "within hours" despite a partial draw-down of forces earlier this week.
Philip Hammond, the British Foreign Secretary, said it remained to be seen whether Russia's withdrawal would lead to a genuine pullout. In Washington John Kerry, the US secretary of state, said Isil was "genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes, and what it does. It is also responsible for crimes against humanity and ethnic cleansing."
He said for the communities targeted "the stakes in this campaign are utterly existential". It was only the second time the US government has used the term "genocide" during a conflict.
In 2004 then secretary of state Colin Powell decided acts committed in Darfur constituted genocide.
Officials in Washington said that, while genocide is a crime under international law, Mr Kerry's "moral statement" did not put the US under any legal obligation to increase its actions against the terrorist group. However, it could put the Obama administration under pressure to take further military action and lend weight to calls for the US to accept more refugees.
Mr Kerry said he was "neither judge, nor prosecutor, nor jury" but the facts must one day go before an international tribunal.