Aspen(CNN) The chief of Britain's foreign intelligence service believes that Russia is losing steam in its invasion of Ukraine, and has lost its ability to spy in Europe "by half" following the expulsion of more than 400 Russian intelligence officers from cities across Europe and the arrest of several deep-cover spies posing as civilians.
Richard Moore, the head of MI6, told CNN's Jim Sciutto at the Aspen Security Forum that since Russia invaded Ukraine in February, European countries have expelled "north of 400 Russian intelligence officers operating under diplomatic cover" across the bloc.
"And we reckon, in the UK, that has probably reduced their ability to do their business to spy for Russia in Europe by half," Moore said. He added that a number of "illegals," or Russian spies operating under deep cover and masquerading as ordinary civilians, have also been exposed and arrested in recent months.
Moore also said he believes Russia could be "about to run out of steam" in Ukraine.
"I think our assessment is that the Russians will increasingly find it difficult to supply manpower material over the next few weeks," he said. "They will have to pause some way and that will give the Ukrainians opportunities to strike back."
"Their morale is still high," Moore said, referring to the Ukrainians. "They're starting to receive increasing amounts of good weaponry." Russia, by contrast, failed significantly in its initial objectives to take Kyiv and overthrow the government there and is largely using "cannon fodder" for its offensives in eastern Ukraine, he said.
Asked whether the war in Ukraine has made Russia a "target rich environment" for the UK and its allies to recruit potential assets, Moore would only say that "it is our hope" that Russians in the intelligence and diplomatic services will "reflect on what they are witnessing in Ukraine" and decide to "strike back against the system" as many did during the Prague Spring in 1968.
"Our door is always open," he said.
Moore also echoed what CIA Director Bill Burns told the forum on Wednesday about Russian President Vladimir Putin's health. "There is no evidence that Putin is suffering from serious ill health," he said. Burns told the forum that the US believes Putin is "entirely too healthy," despite rumors and speculation that he might be sick.
Asked what lessons China has learned from Russia's invasion of Ukraine, especially when it comes to whether Beijing might try to invade Taiwan, Moore said it is too early to tell. But he said that Chinese President Xi Jinping is watching the conflict and how the US and the west are responding to it "like a hawk."
"I think he underestimates US resolve and power," Moore said. "And that might lead him to miscalculate ... particularly over Taiwan." Moore said, however, that he does not believe a war between China and Taiwan is inevitable.
Moore said that MI6 now devotes "more effort to China than any other single subject," but that it is "still a pretty opaque system."
"At one level, understanding Xi Jingping's strategic intent is not difficult," Moore said, citing Xi's stated desire to dominate key technological spaces. "But if you go beneath that strategy in terms of how they implement, how they organize, how they, what their tactical intent is, and then what are the capabilities they're building up, that's a black box."
Moore said it is clear, though, that the Chinese "are helping the Russians over Ukraine by buying their oil." And while they have been "quite conservative about military assistance" to Russia, "I'm sure if they could provide that and get away with it, they would," he said. In terms of the partnership between the countries, "Moscow is very much the junior partner and the Chinese are very much in the driving seat," he added.
Moore also addressed the Iran nuclear deal, telling Sciutto that while he believes the deal should be revived, "I'm skeptical that the Supreme Leader will go for the deal. I think the deal is absolutely on the table and the European powers and the and the administration are very, very clear on that. And I don't think that the Chinese and Russians on this issue would block it. But I don't think the Iranians want it."