The Islamic State terror group could be "enveloped and collapsed" in its main stronghold of Raqqa in Syria and Mosul in Iraq within months, US Defence Secretary Ash Carter has said.
His comments came as UK Defence Secretary Sir Michael Fallon acknowledged that it may have been better to intervene "much earlier" in Syria, where protests in the Arab Spring of 2011 have developed into a devastating five-year civil war.
With US Secretary of State John Kerry due to meet his Russian counterpart Sergei Lavrov in Geneva for the latest in a long series of talks on a possible cessation of hostilities in the country, both Mr Carter and Sir Michael said that Moscow's co-operation was the key to ending the bloodshed.
Mr Carter, who is in London for a peacekeeping conference hosted by Sir Michael, said that the US was willing to give the talks with the Russians "another try", but added: "Our patience is not unlimited."
There was "reason for scepticism" about Russia's claim to be seeking an end to war in Syria, he said.
Turkish President Recep Tayyip Erdogan said earlier this week that he had agreed with President Barack Obama on the sidelines of last weekend's G20 meeting in China on joint operations to drive IS - also known as Isis, Isil or Daesh - out of Raqqa.
Mr Carter confirmed that the US was involved in "joint action" with Turkey to complete the sealing of its border with Syria, which had been a crucial supply route for IS fighters and guns, and said it was working with Syrian democratic forces on "a plan to envelop and collapse Isil's control over Raqqa".
Asked when the operations against Raqqa and Mosul might be completed, Mr Carter said: "As soon as possible - I think months."
He told BBC Radio 4's Today programme: "Our campaign plan calls for the envelopment of both of those cities within months. I think our plan calls for the envelopment - that is the surrounding and collapsing of Isil control over those two key cities, Mosul and Raqqa. That envelopment is under way now and will unfold further in the coming months."
Mr Carter added: "We are for certain going to defeat Isil. We are embarked on that in eastern Syria and in Iraq. But in the end, the violence can't end in Syria until there's a political transition. The Russians are key to that and they need to get on the right side of things and not the wrong side of things.
"Our campaign against Isil, waged by a large coalition ... has three elements. To destroy Isil in Iraq and Syria, because that is the parent tumour of the cancer of Isil. Both the fact and the idea of an Islamic State based on its ideology needs to be destroyed there. We need to do the same wherever this has metastasized or spread - for example Libya and Afghanistan.
"Finally, we all need to protect our own people and our own homelands.
"I'm confident we will succeed. It's difficult and it takes a lot of time, but both in Iraq and Syria that's where it begins. We've been taking specific steps over the last nine months to carry out our campaign plan. It is going exactly as planned for the envelopment of Mosul and Raqqa."
Sir Michael said that ending the civil war would be a vital part in the efforts to deal with the threat from IS.
"None of that can succeed until we get the civil war brought to an end," he told Today.
"That's why we've spent a lot of time this week trying to persuade Russia to use its influence - they virtually run the regime in Syria - to stop the regime bombing its own civilians and using chemical weapons against women and children in Syria. Until the civil war is brought to an end we can't get on and deal with the Daesh terrorists holed up in Raqqa."
Speaking to ITV1's Good Morning Britain, Sir Michael said that the suffering of children affected by chemical weapons attacks in Syria was "a very good example of (why) perhaps we should have intervened much earlier in Syria".
He said: "There's a cost, a price you pay if you don't intervene and put troops or peacekeepers in much earlier in some of these countries.
"But we are now trying to get a political settlement, there were meetings again in London yesterday, but the problem is that the regime that is bombing civilians, bombing hospitals, using chemical weapons - that regime is still backed by Russia and there's been no effort by Russia to actually bring the civil war to a halt.
"Now we have put money aside, we've had countries round the world pledging money to reconstruct Syria but we can't spend that money and get people in until the civil war is brought to an end."