Russia's GRU military intelligence service is seeking to cause "mayhem" across Britain and Europe, the U.K.'s domestic spy chief said on Tuesday, while underlining a growing threat from resurgent al-Qaeda and the Islamic State group as his greatest terrorism concern.

In a wide-ranging speech outlining the current threat picture, Security Service (MI5) Director General Ken McCallum also accused Iran of being behind "plot after plot" on British soil.

McCallum said state threat investigations were up 48% in the last year as Russia and Iran turned to criminals, drug traffickers and proxies to carry out their "dirty work."