The head of MI5 has said his agency has “one hell of a job” to do as the threat from Islamic State has returned while Iran and Russia engage in intensifying efforts to undertake assassination and sabotage plots in the UK.

Ken McCallum said a revival of IS in Afghanistan in particular had brought a resumption of efforts by the Islamist group to export terrorism, and a “bit of an upswing” in Britons seeking to travel abroad to learn from the group.

McCallum said the “worsening threat” from IS and, to a lesser extent, al-Qaida, was “the terrorist trend that concerns me most” and he noted that al-Qaida had “sought to capitalise on conflict in the Middle East” in its calls for violent action.

“Over the last month, more than a third of our top-priority investigations have had some form of connection, of varying strengths, to organised overseas terrorist groups,” he said as he gave a threat update.

Spy chiefs are focused on the revival of IS’s Afghan affiliate, Islamic State Khorasan Province (ISKP), which has grown in strength after the western withdrawal from Afghanistan. It claimed responsibility for the deadly attack in Moscow in March where militants opened fire at a concert, killing 133 people and wounding 140.

Two brothers from Birmingham were sentenced last November for trying to travel to Afghanistan to join ISKP, McCallum noted. They received jail terms of 10 years and eight years.

The spy chief said it was not the case that Israel’s ongoing war with Hamas in Gaza and Hezbollah in Lebanon had led directly to an increase in terrorist plotting in the UK, though he acknowledged there had been “rising public order, hate crime and community safety challenges” that police had had to deal with.

Terror threats tended to develop over a long period, he added. “The ripples from conflict in that region will not necessarily arrive at our shores in a straightforward fashion,” he said, but the UK terror threat remained unchanged at “substantial”, the third level on a five-point scale.

McCallum highlighted that Iran had been behind “plot after plot” in the UK in the past two years. Five new Iran-backed plots have been uncovered by MI5 and police this year, taking the total since January 2022 to 20.

McCallum said Iranian state actors made “extensive use of criminals as proxies”, to try to carry out threats and intimidation largely directed against dissidents and individuals perceived as a threat to the Tehran regime.

He said MI5 was alive to the possibility that Iran “could, in principle, try to repurpose” that effort to focus on other targets in the UK if Tehran felt that Britain had become a party to the conflict in the Middle East by supporting Israel in its anticipated retaliation to last week’s ballistic missile attack.

He also said Russia’s GRU military intelligence agency was engaged in “a sustained mission to generate mayhem on British and European streets”, noting that there had been arson and sabotage plots, also relying on criminal networks to carry out disruptive attacks because most or all of the country’s embassy-based spies had been kicked out.

Taken together, the number of MI5’s state-based investigations, including China as well as Russia, Iran and others, had risen by 48% in the past year. Russian activity had stepped up again after a chaotic period after the invasion of Ukraine, when 750 diplomats had been expelled across Europe.

McCallum said that meant the spy agency was dealing with terror threats alongside “state-backed sabotage and assassination plots”, and he observed that “MI5 has one hell of a job on its hands”.

McCallum also said the number of terror cases that involved MI5 investigating under 18-year-olds was continuing to grow, particularly where extreme rightwing threats were involved online.

“Sadly, 13% of all those being investigated by MI5 for involvement in UK terrorism are under 18,” the spy chief said. “That’s a threefold increase in the last three years. Extreme rightwing terrorism in particular skews heavily towards young people, driven by propaganda that shows a canny understanding of online culture.”

For several years, MI5 has found itself investigating teenagers’ activity online, and three years ago said it had realised that the subject of one of its inquiries into neo-Nazi activity was 13.

The number of late-stage terror plots disrupted since March 2017 had increased to 43, McCallum said. The last time the spy chief gave a comparable figure it was 37 in November 2022, meaning six plots have been prevented in nearly two years.