Russian President Vladimir Putin has suffered an "epic fail" in Ukraine and his forces may have to "pause" in the coming weeks, the head of MI6 has said.
Richard Moore, the chief of the UK Secret Intelligence Service (MI6), said the war was "obviously not over" but the Russian invaders may be starting "to run out of steam".
Speaking at the Aspen Security Forum in the US, he said: "I think he has suffered a strategic failure in Ukraine. It is obviously not over.
"He has obviously made, and the Russian forces have made, some incremental progress over recent weeks and months but it is tiny amounts.
"We are talking about a small number of miles of advance. When they take a town, there is nothing left. It is obliterated.
"And I think they are about to run out of steam. I think our assessment is that the Russians will increasingly find it difficult to supply manpower over the next few weeks.
"They will have to pause in some way and that will give Ukrainians opportunities to strike back. Their morale is still high. They are starting to receive increasing amounts of good weaponry."
Mr Moore believes a Ukrainian fightback could spread benefits across Europe.
He said: "It is important I think to the Ukrainians themselves that they demonstrate their ability to strike back. I think that will be very important for their continuing high morale.
"I also think, to be honest, it will be an important reminder to the rest of Europe that this is a winnable campaign by the Ukrainians."
Watch: Constantly on the move, members of the Ukrainian army's mobile tank repair unit drive along the frontline in Donbas, eastern Ukraine.
Mr Moore told the US security conference, that half of all the Russian spies operating under diplomatic cover around Europe have been removed since the start of the war in Ukraine.
The invasion has triggered widespread sanctions against the Russian regime.
Mr Moore said the "pretty concerted effort" against Russia has led to the expulsion of "something roughly north of 400" intelligence officers who were operating under diplomatic cover.
"We reckon in the UK that has probably reduced their ability to do their business to spy for Russian in Europe by half," he added.
China watching "like a hawk"
Mr Moore thinks "it is too early to tell what lessons they (China) will draw from Putin's misadventures in Ukraine" but China's President "Xi Jinping is watching this like a hawk".
This makes it "essential" that "we tough it out on Ukraine", keep going through this winter and help the Ukrainians to win and negotiate from a position of significant strength, he suggested.
Russia and China are said to have a relationship that is based on the idea of "no limits", the conference heard.
According to Mr Moore, despite being "quite conservative about military assistance", the Chinese are helping the Russians over Ukraine by buying their oil.
The chief of MI6 added: "They are right on the front foot of beating the Russian drum and selling the Russian narrative around Ukraine and doing it without any sense of irony.
"This is a country that spends a lot of time banging on about sovereignty and territorial integrity and here is the most egregious example of someone tearing that up in Europe and the Chinese keep on selling their snake oil around the world.
"It feels pretty tight but it is not an equal partnership. Ukraine has made it less equal.
"Moscow is very much the junior and the Chinese are very much in the driver's seat."