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Russia is "about to run out of steam" in Ukraine, the head of MI6 has said.
Richard Moore said President Vladimir Putin's soldiers will "have to pause" as they find it hard to find more troops and equipment to send to the frontline in eastern Ukraine in the coming weeks.
This will enable the Ukrainian military to "strike back" in what the UK spymaster said is a "winnable war".
Speaking at a security forum in Aspen, Colorado, Mr Moore also commented on the state of Mr Putin's health amid rumours he had been suffering serious sickness such as cancer.
"There is no evidence that Putin is suffering from serious ill health," he said.
Speaking at the same security forum on Wednesday, William Burns, the director of the CIA, also played down speculation about Mr Putin being ill, saying he is "entirely too healthy".
On the war in Ukraine, the MI6 spy chief said Russia had suffered "epic fails" in its initial objectives, which he listed as toppling the Ukrainian government, capturing Kyiv, and sowing division within the NATO alliance.
Russia's focus has shrunk to the east of Ukraine, where it is waging a bloody war of attrition.
Mr Moore said: "I think they are about to run out of steam.
"I think our assessment is that the Russians will increasingly find it difficult to find manpower, material, over the next few weeks, they will have to pause in some way and that will give the Ukrainians opportunities to strike back.
"Their morale is still high, they are starting to receive increasing amounts of good weaponry.
"It is important for the Ukrainians themselves that they demonstrate their ability to strike back. That will be important for their continuing high morale."
Mr Moore said it was a significant reminder for the rest of Europe that "this is a winnable campaign by the Ukrainians", as allies brace for what will be a "pretty tough winter" while the impact of Europe trying to wean itself off a dependence on Russian energy is felt.
Offering a quote from the TV series Game Of Thrones, Mr Moore said: "Winter is coming and clearly in that atmosphere, with the pressure on gas supplies, we are in for a tough time."