The director general of Britain's MI5 domestic intelligence service warned on Tuesday that Russia remained "aggressor number one" amid increasing threats from foreign states.
Delivering an annual speech from Thames House in London, Ken McCallum told attendees that MI5 was currently "devoting more resources to countering state threats than during the Cold War."
"The Russian military and intelligence services are once again escalating their attempts at hybrid warfare, being careful to stay below the NATO Article 5 threshold, which would be foolish for them to cross," he added.
A number of states have accused Moscow of being behind hybrid attacks on European territory in recent months, including Germany and Estonia. Berlin blamed Russia for a drone attack at Leipzig/Halle airport in August, while Tallinn said that Russian special services had ordered an arson attack targeting defence firm Milrem Robotics, also in August.
According to McCallum, such incidents are part of a Russian plan to "unsettle" European nations as it continues to wage its all-out war in Ukraine, which began in February 2022 after Moscow launched its full-scale invasion of the country.
McCallum said the ramp up in alleged Russian hybrid threats had come as Russia's military progress in Ukraine was "minimal and horrifically costly," adding that it was turning to proxies to carry out attacks.
"With its traditional covert footprint eviscerated by almost 1,000 expulsions since the invasion, the Kremlin's global toolkit is weak. So, we see them scrambling to pay criminals to do their bidding," he told the audience.
British Prime Minister Andy Burnham has said the UK would continue to support Ukraine "100%" despite warnings of "consequences" from Russia. Last week, Burnham again reiterated his "complete solidarity" with Kyiv and stressed that his government would go further with sanctions on Russia.
Iran
McCallum also addressed Iran in his speech, calling it another state "keen on subcontracting its UK violence" and saying it was looking to "expand" its hybrid campaign.
"Today, we continue to see familiar Iranian intent, using criminal proxies to try to intimidate oppositionists, Iranian media organisations, and Israeli sites," he said.
Over the last year, McCallum said Iran-linked operations had led to more than 50 arrests.
He added that much of the aggression had targeted British Jewish communities, but that Iran was also looking to go after "harder targets, like military sites."
His comments come after Burnham told parliament on Monday that an incident at a UK airbase that houses part of the US Air Force was "almost certainly directed" by Iran's Revolutionary Guard Corps (IRGC).
On 27 September, five men were arrested near RAF Fairford in Gloucestershire, southwest England, on suspicion of explosives offensives, before being further arrested on suspicion of preparing a terrorist act. They were later released on police bail.
Police later arrested another man, a 25-year-old dual UK-Iranian national, in London on suspicion of preparation of terrorist acts related to the incident. A 22-year-old British man was then arrested on 6 October, also in London and on suspicion of preparation of terrorist acts. Both were released on police bail.
According to UK Counter Terrorism Policing, all seven men remain on police bail as specialist teams "continue to pursue all lines of enquiry."