Although the war was “obviously not over”, the Russian invaders may be starting “to run out of steam” said Richard Moore, the chief of the UK Secret Intelligence Service, MI6.
“I think he has suffered a strategic failure in Ukraine. It is obviously not over," Mr Moore told the Aspen Security Forum in the US.
“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.”
With winter coming and the pressure on gas supplies, “we are in for a tough time”, said Mr Moore, who believes a Ukrainian fightback could spread benefits across Europe.
“It is important, I think, to the Ukrainians themselves that they demonstrate their ability to strike back," he said. "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.”
Mr Moore said there was no evidence that Mr Putin was suffering from serious ill health and that going into the invasion “the reality of what they were about to encounter was just not being briefed up” to the Russian leader.
The invasion has prompted 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 said.
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 said.
Russia and China are said to have a relationship that is based on the idea of “no limits”, the conference heard.
Despite being “quite conservative about military assistance”, the Chinese are helping the Russians with Ukraine by buying their oil, Mr Moore said.
“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 …" he said.
“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.”
The spy chief also voiced doubt that a landmark 2015 nuclear accord with Iran can be revived, saying Supreme Leader Ayatollah Ali Khamenei remained opposed despite marathon diplomacy with the United States.
"If we can get a deal it's probably the best means still available to constrain the Iranian nuclear program. I'm not convinced we're going to get there," said the MI6 chief.
"I don't think the supreme leader of Iran wants to cut a deal. The Iranians won't want to end the talks either so they could run on for a bit," Mr Moore said in a live interview in the US state of Colorado, in what was billed as his first public speaking appearance abroad.