In a rare public speech abroad, the head of the Secret Intelligence Service (SIS, also known as MI6), Richard Moore, also said that while Iran had been affected by the degrading of allied militias in the Middle East, its nuclear programme "threatens us all".

"We have recently uncovered a staggeringly reckless campaign of Russian sabotage in Europe, even as Putin and his acolytes resort to nuclear sabre-rattling to sow fear about the consequences of aiding Ukraine and challenge Western resolve," Moore said in a speech in Paris.

He did not disclose details as to the nature of the sabotage but said such "activity and rhetoric" was "dangerous and beyond irresponsible".

Moore acknowledged the cost of Western support for Ukraine, which has been fighting Russia's full-scale invasion since February 2022.

But he warned the "cost of not doing so would be infinitely higher".

"If Putin is allowed to succeed in reducing Ukraine to a vassal state, he will not stop there," said Moore.

"If Putin succeeds, China would weigh the implications, North Korea would be emboldened and Iran would become still more dangerous," he said.

But, Moore continued: "We have no doubt that our Ukrainian friends have the will to win."

British and French intelligence services had been "critical" in informing decisions made by their respective governments, "so they can navigate successfully Putin's mix of bluster and aggression", he said.

Moore said Russia was relying on support from China, Iran and North Korea, whose military assistance for Moscow, according to the West, now extends to the deployment of troops.

"Putin... is jeopardising Russia's future, pouring vast sums into his military machine and squandering tens of thousands of lives -- Russian and now North Korean -- in his catastrophic conflict," Moore said.

- 'Never seen a more dangerous world' -

Moore meanwhile said that Iran's nuclear ambitions posed a major global security threat even after the blows dealt by Israel to Tehran-backed militant groups Hamas in the Gaza Strip and Hezbollah in Lebanon.

"Iran's allied militias across the Middle East have suffered serious blows. But the regime's nuclear ambitions continue to threaten all of us."

Western nations have long accused Iran of seeking a nuclear weapon -- a charge denied by Tehran -- and fear the upsurge in tensions with Israel over Gaza and Lebanon will only accelerate that process.

After authorities in the United States and Europe warned of the risk to Iranian exiles of killings and even kidnappings, Moore said "the Iranian regime maintains its efforts to eliminate dissidents at home and abroad".

Moore, a former British ambassador to Turkey, has combined a career in the secret service and diplomacy and is the first head of M16 -- a position traditionally known as "C" -- to be active on social media.

"In 37 years in the intelligence profession, I have never seen the world in a more dangerous state," he commented.

In the highly unusual event organised by the UK embassy in Paris to celebrate security and intelligence ties, Moore was introduced by his French counterpart, Nicolas Lerner, head of France's Directorate-General for External Security (DGSE).

"The possible nuclear proliferation in Iran" constitutes "one of the threats, if not the most critical threat in the coming months", said Lerner.

"The exchange of intelligence will be crucial to enable our authorities to make the right decisions and define the right strategies," the French intelligence chief added.