UK spy chief: Taiwan's future at stake in Ukraine war

And that was "one reason why it's so essential we tough it out going into this winter and we continue to help Ukraine to win, or at least negotiate from a position of significant strength," Moore said.

Xi had an "ingrained narrative of Western weakness", Moore added.

"I think he [Xi] misunderstands US resolve and power and that could lead him to miscalculate over Taiwan", Moore said.

"It's really important [for Ukraine to win] as president Xi looks at what he may or may not do with Taiwan and sees what can happen with a misjudged invasion," the MI6 chief said.

Moore, a former diplomat who took up his MI6 post two years ago, spoke at the Aspen Security Forum in Colorado in his first-ever public remarks abroad.

Winter would be tough, because European countries would come under "pressure" to abandon Ukraine due to Russian gas cut-offs, he said.

But Russian president Vladimir Putin's offensive in east Ukraine was slowing to "incremental progress" and his forces would soon have to pause to regroup, giving Ukraine a chance to strike back, Moore said.

"It [a Ukrainian counter-offensive] would be an important reminder to the rest of Europe that it's a winnable war," he said.

Russia had already lost 15,000 soldiers — more than in Afghanistan in Soviet times, he noted, and Putin got a "bloody nose" trying to capture Kyiv.

His Ukraine war was also a "strategic failure" in terms of broader Russian foreign policy, Moore said.

It had prompted "tectonic shifts" in the Western military allegiance, such as Germany agreeing to export arms and Finland and Sweden joining Nato, he said.

"Sweden just gave up 200 years of neutrality," he added.

Europe had also ejected "roughly half" of all Russian spies working under diplomatic cover in its capitals, Moore noted, referring to the 530 recent expulsions by EU and allied states since April.

"We in the UK estimate that reduced their ability to spy in Europe by half," he said.

Russian "illegals" — spies under non-diplomatic cover — were also being exposed as in recent cases in Spain and the Netherlands, he also said.

One reason why Putin charged into Ukraine was because his own intelligence chiefs were too scared to tell him what he would really face there, Moore said.

It "doesn't pay to speak truth to power" in the Kremlin and Putin "just wasn't being briefed up", Moore said.

But Putin also thought he still had friends on the world stage — including China and Iran.

China has been helping Russia by buying its oil and spreading its war propaganda, Moore said.

"They've [the Chinese] been quite conservative about military assistance [to Russia] but I'm sure if they could provide that and get away with it, they would," he added.

The West ought to "listen hard" when Xi says his partnership with Putin has "no limits", as he did before the Ukraine invasion, Moore said.

But "this is not an equal partnership and Ukraine has made it less equal," he added. "China is very much in the driving seat and Russia is the junior partner," he said.

The West might not share the same values as Gulf Arab states, but it should still "work our friends in the Gulf" to help weaken Iran, Moore added.