Half of all the Russian spies operating under diplomatic cover around Europe have been expelled since the start of the war in Ukraine, the chief of MI6 has told a US security conference.

Richard Moore, who heads British foreign intelligence, said the expulsions of about 400 Russian diplomats from countries in continental Europe, including France and Germany, had dramatically reduced the Kremlin’s espionage capabilities.

Speaking at the Aspen security conference, he said western intelligence agencies had made “pretty concerted” efforts to disrupt Russian spying networks since the invasion.

“So across Europe, roughly half – at last count something north of 400 Russian intelligence officers operating under diplomatic cover – have been expelled,” Moore said. “That’s probably reduced their ability to do their business to spy for Russia in Europe by half.”

It is the first time that MI6 has gone public with its estimate of the impact of the coordinated expulsions in response to the invasion in February. Although the figure of 400, across a wide range of nations, had been previously tallied, the proportion had not.

Russian spies, as with intelligence officers of almost every major country, typically pretend to be undertaking cover jobs at their country’s embassy. Only a handful of long-term spies – so-called illegals – pretend to be ordinary citizens, operating outside the diplomatic framework.

Germany kicked out 40 Russian diplomats in April, and France 41. Britain is one of the few countries not to have expelled any diplomats accused of spying, although that is largely because the UK told 23 to leave in 2018 in the aftermath of the Salisbury poisonings and it is not believed any have been replaced.

Moore said two illegals had been unmasked, including a Russian pretending to be Irish-Brazilian who had tried and failed to secure an internship at the international criminal court in The Hague in April. A Brazilian court has since sentenced Sergey Cherkasov to 15 years in jail for using false identity documents.

Moore said he believed the war in Ukraine was “a winnable campaign” for Kyiv and that Russia would “increasingly find it difficult to supply manpower and material over the next few weeks” as the war headed towards a critical phase before the weather turned.

The spy chief appeared to hint that Ukraine, with the support of growing quantities of western weapons, would attempt to stage a counterattack in the coming weeks. “It’s important, I think to the Ukrainians themselves, that they demonstrate their ability to strike back,” he said.

Moore said he agreed with his counterpart at the CIA, Bill Burns, that there was “no evidence that Putin is suffering from serious ill health”. There had been speculation in the early phases of the war that the Russian president had cancer or another serious illness, and had been taking steroids.

Moore also agreed with the CIA chief that China was not supplying Russia with weapons because Beijing was nervous about being hit with western economic sanctions, but he said he thought that “if they could provide weapons and get away with it, they would”.

He said China was “going into overdrive” to work out what lessons it could draw from the Ukraine war and the western response so far. “It’s too early to tell what lessons they will draw from Putin’s misadventures,” Moore said, and he repeated a previous warning that Beijing should not underestimate western resolve to protect Taiwan from any attempt at forced reunification.