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: "Although we and our partners try our utmost, we know that we cannot hope to stop everything."
Parker, who has led Britain's domestic intelligence agency since 2013, was speaking late on Thursday, a day after Islamist gunmen stormed the offices of French satirical magazine Charlie Hebdo in Paris and killed 12 people.
He said it was too early to judge the details or the origin of the attack, 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 Royal United Services Institute (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".
The Islamic State group in Syria and Iraq is a well-known threat, targeting Britain and seeking to radicalize Britons, with Parker saying an estimated 600 have gone there to fight.
But 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.
Raffaello Pantucci, an expert in international security at RUSI, said Parker was making clear in his speech that despite the apparent changing nature of recent attacks, the old threats do not go away.
"So while we have seen a surge in this kind of lone actor-type thing, we've also still got a lot of people trying to plan big attacks," he told AFP.
"The fact that we haven't had a successful one does not mean that people aren't plotting to do that."
Case for more powers
After laying out the threats, Parker made a fresh call for more powers for the security services — an issue that is of ongoing and heated debate in Britain, as elsewhere.
"My sharpest concern as director general of MI5 is the growing gap between the increasingly challenging threat and the decreasingly availability of capabilities to address it," he said.
A previous bill to force companies to keep data about people's online conversations, social media activity, calls and texts for a year was dropped after opposition from the junior coalition partners in the government.
But another law was published in November that would oblige Internet service providers to hand over data on Internet Protocol (IP) addresses to trace individual smartphone and computer users.
Malcolm Rifkind, who chairs parliament's Intelligence and Security Committee scrutiny body, said the Paris attack gave "added weight" to the case for new surveillance powers to track militants.
"In the modern world they communicate through the Internet, through email, through social messaging and all the technical ways that we are aware of," the former foreign and defence secretary told BBC radio.
"So our intelligence agencies, acting under law, acting on good reasonable grounds, with authority, have to have the power to intercept particularly international communications that might be relevant."
French Prime Minister Manuel Valls made a similar point Friday, saying: "It will be no doubt necessarily to take new measures" to respond to the terror threat.
France has invited ministers from other European countries and the United States to Paris on Sunday to discuss their common security challenges.