MI5 has had to “pare back” its counter-terrorism focus because of the growing threat from Russia and other hostile states, the security agency’s boss has said.

Ken McCallum, the director general of MI5, said the agency has to look at its “finite” resources and make “uncomfortable choices”.

“We now face much, much more aggression from nation states,” he said in an interview recording obtained by the Times. “In effect, we had the 20- to 30-year holiday from that kind of big player, sophisticated states in serious conflict with each other. It’s back, I’m afraid.

“And so there is something quite profound about that … we’ve had to make some of those uncomfortable choices in recent years … how could we pare back a bit on the amount of our capacity we are spending on countering terrorism in order to be ready to meet these somewhat grander and more sophisticated, in some respects, threats from nation states.”

He made the comments in an interview with Simon Case, the outgoing cabinet secretary, for a civil service podcast Case is making, the newspaper reported.

McCallum said: “We’ll be looking at how much of our finite capacity can we spend on state activity from Russia or Iran or China, how much do we have to spend on various forms of terrorism, but also how much of our capacity can we spend on things that we think might be a threat tomorrow morning and how much do we have to keep back to plan for how we are capable to do what the nation needs us to do in five years’ time?

“We need to invest in new technology, new skills. So you always have some version of these uncomfortable choices.”

The MI5 boss recently warned that Russia is intent on causing “mayhem” on UK streets and that Iranian-backed plots that pose “lethal threats” to British people are ramping up at an “unprecedented pace and scale”.

In a speech in October, the security agency’s director general said Britain should “expect to see continued acts of aggression here at home” from Russia, with its military intelligence agency, the GRU, on “sustained missions”, adding: “We’ve seen arson, sabotage and more.”

The number of state threat investigations run by MI5 has “shot up by 48%” in the last year, and 13% of people being investigated by the security agency for involvement in UK terrorism are under 18, he said at the time.

He called the rise in the number of children being investigated for terrorism in the UK “staggering” and warned of “canny online memes” drawing them into extreme rightwing ideologies.

A Cabinet Office spokesperson said: “This innovative conversation series was provided on the civil service learning site to offer insight into the experience of senior leaders from across the civil service, to allow other civil servants to learn and develop, as is so vital throughout their careers.

“The comments made by Ken McCallum, director general of MI5, reflect those he has previously made publicly. In his October threat update, he talked about the challenges of prioritisation.”