Security Service chief Andrew Parker was referring in particular to a group of fighters known as Khorasan, sources said, who have been targeted by US warplanes in strikes in northwest Syria.

He warned that intelligence agencies needed more powers, in particular to intercept communications, but added: "We know that we cannot hope to stop everything."

Parker was speaking late Thursday, a day after gunmen stormed the offices of French satirical magazine Charlie Hebdo in Paris and killed 12 people.

He said the origins or details of the attack remained unclear, but it was a "terrible reminder of the intentions of those who wish us harm".

Speaking of the broader threats, he said the number of "crude but potentially deadly plots" had increased, citing last year's attacks in Canada and Australia, and were particularly hard to detect.

Meanwhile the "complex and ambitious plots" carried out by Al-Qaeda in the past still existed, he told the RUSI defence and security think-tank in London.These included "attempts to cause large scale loss of life, often by attacking transport systems or iconic targets".

In August, Britain raised the national threat level to "severe" -- the second-highest of five levels -- meaning that an attack is considered "highly likely".

This was primarily due to the rise of the so-called “Islamic State” militant group in Syria and Iraq, where an estimated 600 Britons have gone to fight, Parker said.

The MI5 chief added: "We know, for example, that a group of core Al-Qaeda terrorists in Syria is planning mass casualty attacks against the West."

He is understood to be referring to the Khorasan group, which had been an obscure cell until US intelligence officials called attention to it in September.

US officials say the group includes Al-Qaeda operatives from Afghanistan and Pakistan who made their way to Syria, although experts caution they are not a distinct group but are part of Al-Nusra Front, Al-Qaeda's Syrian branch.