In an interview with The Guardian newspaper published Tuesday
, Andrew Parker, director-general of MI5, said that the covert threat from foreign countries, most notably Russia, was rising at a time when the threat of radical Islam drew the most attention.
Russia was "using its whole range of state organs and powers to push its foreign policy abroad in increasingly aggressive ways -- involving propaganda, espionage, subversion and cyber attacks," he said, in the first newspaper interview with an incumbent MI5 chief in the service's history.
"Russia is at work across Europe and in the UK today. It is MI5's job to get in the way of that."
Besides Russia's high-profile activities in Ukraine and Syria, it was also responsible for "high-volume activity out of sight with the cyber-threat," he said.
"Russia has been a covert threat for decades. What's different these days is that there are more and more methods available."
He said that Moscow "increasingly seems to define itself by opposition to the West and seems to act accordingly."
Moscow: Statements 'not true'
Kremlin spokesman Dmitry Peskov rejected Parker's assertions in a press conference in Moscow Tuesday.
"These words are not true, we cannot agree with them," he told journalists, according to state-run news agency TASS.
The Russian embassy in London responded tartly, tweeting on its official account that it was "saddened to see a professional trapped (in) his own propaganda-created world."
Accompanying the message was an image of the poster for the 1966 US comedy film, "The Russians Are Coming, the Russians Are Coming."
Russia has faced scrutiny for alleged covert actions recently, with the Obama administration accusing the Kremlin last month of hacking Democratic National Committee emails
about next week's US elections.
The Department of Homeland Security and Office of the Director of National Intelligence said in a joint statement that the release of the hacked emails was "intended to interfere with the US election process," adding that such actions were "not new" for Moscow.
"The Russians have used similar tactics and techniques across Europe and Eurasia, for example, to influence public opinion there," the statement said.
Russia rejected the hacking allegations, with Foreign Minister Sergey Lavrov labeling them "ridiculous."
MI5 head: 'There will be terrorist attacks'
At a speech in London on Monday, Parker also addressed radical Islamic terror, in particular the threat posed by ISIS.
During his keynote address to the Royal Society's annual diversity conference, he said UK authorities had disrupted 12 jihadist terror plots on British soil since June 2013.
In comments following the speech, he described ISIS as "an enduring threat, here to stay, and ... at least a generational challenge."
"MI5 and the intelligence agencies have good defenses because of the investment made in our capabilities," he said. "We will find and stop most attempts to attack us, but not all."
He told The Guardian that the radical Islamist threat to the UK came from three vectors. There were are 3,000 potentially "violent Islamic extremists in the UK, mostly British"; ISIS members in Syria and Iraq working to incite terror attacks in the UK; and the threat of ISIS propaganda online, working to radicalize people in the UK.
A 'dozen' plots disrupted
He said the dozen plots disrupted in three years showed terrorist activity at a level "higher than I have experienced in the rest of my career -- and I've been working at MI5 for 33 years."
"There will be terrorist attacks in this country," he told the newspaper. "The threat level is severe and that means likely."
Parker said that aside from Islamist terror and the actions of hostile foreign states, the third major threat posed to the UK was terrorism in Northern Ireland, carried out by dissident Irish republicans.
He also told the newspaper that Brexit would not negatively impact the cooperation between British security services and their European counterparts, and said that MI5 would grow by a quarter to total 5,000 officers over the next five years.