A group of core al Qaida terrorists in Syria is planning "mass casualty attacks" against the West, the head of MI5 has warned, in a stark reminder that the threat to the UK continues to stretch beyond Islamic State (IS).
As dramatic events surrounding the terrorist attack at the offices of Charlie Hebdo magazine in Paris continue to unfold, Andrew Parker, director general of the Security Service, said transport networks and iconic landmarks were among Western targets of "complex and ambitious plots" by Syria-based extremists.
Aviation bomb plots and Mumbai-style shootings in crowded places are thought to be among plans being developed by the shadowy group, which has Europe in its sights. It is also understood the organisation in question is the so-called "Khorasan" cell, which is made up of veteran jihadists sent to Syria by al Qaida leader Ayman al-Zawahiri.
Khorasan is embedded within al Qaida's Syrian branch al-Nusra Front, which is known to include radicalised Britons who have travelled to the war-torn country to fight.
Addressing about 70 members of the Royal United Services Institute (RUSI) at MI5 headquarters Thames House, Parker said: "We still face more complex and ambitious plots that follow the now sadly well-established approach of al Qaida 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 Qaida terrorists in Syria is planning mass casualty attacks against the West." He added: "Al Qaida continues to provide a focus for extremists to plot terrorist attacks against the West. British Islamist extremists still travel to South Asia, the Arabian Peninsula and other theatres to try to obtain terrorist training."
Khorasan has been described as a small group made up of around 50 fighters, including expert bomb makers and high-ranking members of al Qaida who moved to Pakistan following the 2001 invasion of Afghanistan.
US officials said they had been sent to Syria not to fight the government of President Bashar Assad but to "develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations".
In September, US military forces carried out air strikes against Khorasan after intelligence reports suggested that the group was in the final stages of plans to launch major attacks against Western targets including the United States. Parker said around 600 extremists are now among many Britons who have travelled to Syria - higher than previous estimates of 500.
The Director General said a significant proportion has joined Islamic State, the extremist group that has taken over large swathes of Iraq and Syria and is behind a series of beheadings of hostages in recent months. "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 through which many of us now live our lives. By these means it spreads its message of hate directly into homes across the UK - both to those seeking it and those who may be susceptible to its distortion and glamorisation of horrific acts."
The warning came on the same day the an Islamic State fighter praised Wednesday's attack in the French capital, telling Reuters the raid was revenge for insults against Islam. "The lions of Islam have avenged our Prophet," said Abu Mussab, a Syrian fighting for IS.
On Thursday, European sources close to the investigation told Reuters that one of the two brothers suspected of carrying out the shooting visited Yemen in 2011 to train with al Qaeda militants. Said Kouachi, 34, was reportedly in Yemen for several months training with Al Qaeda in the Arabian Peninsula (AQAP).
In London, the Director General disclosed that in recent months three UK terrorist plots which would have led to deaths have been foiled by MI5 and its intelligence partners at MI6 and GCHQ. It is understood the plots were a mix of mass casualty attacks and lone-wolf style killings as seen on the streets of Woolwich when Fusilier Lee Rigby was killed in 2013.
Outside Iraq and Syria, the Security Service believes that since October 2013 there have been more than 20 terrorist plots either directed or provoked by extremist groups in Syria. Among them were the deaths of four people shot in Brussels last May by a French returnee from Syria, while in Canada, a soldier was killed in a hit and run attack and another shot dead outside the parliament building.
Other attacks have been foiled, Parker said, such as in early 2014 when police in France seized improvised explosive devices from a flat linked to another Syria returnee. "But we cannot be complacent," he said. "Although we and our partners try our utmost we know that we cannot hope to stop everything."
The Director General said the UK threat level, ramped up to severe from substantial in August, meaning an attack is highly likely, is unlikely to abate for some time. Parker also argued the case for improving intelligence agencies' capabilities to access terrorist communications.
He said his "sharpest concern" as head of MI5 is the "growing gap between the increasingly challenging threat and the decreasing availability of capabilities to address it".
His comments come after a report by the Intelligence Security Committee into the death of Fusilier Rigby criticised social networking giant Facebook for failing to flag extremist communications made by one of the soldier's killers.
It is understood security officials want to see increased co-operation from Internet firms, as well as increased powers to demand access to communications when they do not comply. "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," the Director General said.