Isil is carrying out genocide, warns Kerry
US declaration puts fresh pressure on Obama to take military action
The United States has declared that the slaughter of Christians, Yazidis and Shiites by Islamic State of Iraq and the Levant (Isil) 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.
British Foreign Secretary Philip Hammond said it remained to be seen whether Russia's withdrawal would lead to a genuine pull-out.
In Washington, US Secretary of State John Kerry 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 Isil.
However, it would 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.
On Wednesday, a State Department spokesman Mark Toner said: "Acknowledging that genocide or crimes against humanity have taken place in another country would not necessarily result in any particular legal obligation for the United States."
Isil militants have swept through Iraq and Syria in recent years, seizing control of large swathes of territory with an eye toward establishing jihadism in the heart of the Arab world.
The group's videos depict the violent deaths of people who stand in its way.
Opponents have been beheaded, shot dead, blown up with fuses attached to their necks and drowned in cages lowered into swimming pools, with underwater cameras capturing their agony.
US President Barack Obama has ordered air strikes against the group but has not made any large commitment of US troops on the ground.
"It may strengthen our hand getting other countries to help. It may free us against some (legal) constraints, but the reality is that when you are fighting somebody, you don't need another reason to fight them," said Jon Alterman, director of the Middle East programme at the Center for Strategic and International Studies, a think-tank.
Isil militants have exploited the five-year civil war in Syria to seize areas in that country and in neighbouring Iraq.
On-again, off-again peace talks got under way this week in Geneva in an effort to end the civil war, in which at least 250,000 people have died and millions have fled their homes to Turkey and further north to Europe.
In his first public remarks since ordering the withdrawal, Putin for the first time put an price tag on the Russian operation, saying most of the money - 33 billion roubles (€375m) - came from the defence ministry's war games budget.
There would be other costs, he said, in order to replace ammunition and weapons as well as to make repairs.
Russian air strikes against Isil, Al Nusra and other terrorist groups would press on, he said, as would a wide range of measures to aid Syrian government forces including helping them plan their offensives.
Putin said he did not want to have to escalate Russia's involvement in the conflict again after the draw-down and was hoping peace talks would be successful.
But he made clear Russia could easily scale up its forces again.
"If necessary, literally within a few hours, Russia can build up its contingent in the region to a size proportionate to the situation developing there and use the entire arsenal of capabilities at our disposal," he said.
In a thinly disguised warning to Turkey and others, he said Russia was leaving behind its most advanced S-400 air defence system and would not hesitate to shoot down "any target" which violated Syrian air space.