Video report by UK Editor Rohit Kachroo
A group of al-Qaida terrorists in Syria is planning "mass casualty attacks" against the West, the head of MI5 has warned.
Andrew Parker, the Director General of the security service, said the UK still faced the threat of "complex and ambitious plots" by extremists who wanted to "cause large scale loss of life" by targeting transport networks and iconic landmarks.
Security sources have told ITV News that spies have foiled four terrorist plots against the UK in the past year that would have "certainly" resulted in deaths.
Mr Parker said although security agencies would "try our utmost we know that we cannot hope to stop everything".
He said the terrorist attack which left 12 people dead in Paris yesterday was a "terrible reminder of the intentions of those who wish us harm".
The threat to the UK went beyond Islamic State, the Director General said, with al-Qaida terrorists still posing a significant danger to the UK.
The group mentioned is believed to be the so-called "Khorasan" cell, an al-Qaida branch made up of veteran jihadists sent to Syria by the organisation's leader Ayman al-Zawahiri.
Addressing members of the Royal United Services Institute (RUSI) at MI5 headquarters Thames House, Mr Parker said: "We still face more complex and ambitious plots that follow the now sadly well established approach of Al Qaeda and its imitators - attempts to cause large scale loss of life, often by attacking transport systems or iconic targets.
"We know, for example, that a group of core Al Qaeda terrorists in Syria is planning mass casualty attacks against the West."
Describing the "ongoing and increasing challenge from Syria", Mr Parker said around 600 extremists - including Britons - are known to have travelled to Syria to join groups such as Islamic State, higher than previous estimates of 500.
Mr Parker said more than 20 terrorist plots have either been directed or provoked by extremist groups in Syria since October 2013, citing attacks in Brussels, Ottawa, Sydney and yesterday's attack against Charlie Hebdo magazine in Paris.
The Director General also spoke of the need to improve intelligence agencies' abilities to access communications.
He said his "sharpest concern" as head of the service was the "growing gap between the increasingly challenging threat and the decreasing availability of capabilities to address it".
"Despite its medieval tactics, ISIL is a terrorist phenomenon of the modern age," he said.
"It makes full use of the modern social media and communications methods to enable it to spreads its message of hate directly into home across the UK.
"The dark places from where those who wish us harm can plot and plan are increasing.
"We need to be able to access communications and obtain relevant data on those people when we have good reason to do so."