The director general of the British counterintelligence service MI5, Andrew Parker, characterized the attack on the French satirical newspaper "Charlie Hebdo" as a "terrible reminder" that someone "wishes us harm".
He added that the ever-present terrorist threats have become more complex due to the events in Syria.
Parker used a rare public address, from MI5 headquarters in London, to point out that the number of Britons going to Syria to join extremists there was steadily increasing, stressing that when they return to the country they could pose a major threat.
He said that so far about 600 Britons have gone to the Middle East, of which a "significant number" have joined the extremist group Islamic State.
Parker, in his second public address to date, without mentioning Islamic State by name, said the group threatens Britain in three ways: killing Britons in Syria, using Syria as a base to send its members to Britain and other countries to carry out attacks , and using propaganda to provoke the British in the United Kingdom to carry out attacks.
He stated that the Islamic State is adept at using social media, which means, as he assessed, that it can virtually reach every home in Britain and thus propagate attacks.
Due to the threat posed to Britain by the situation in Syria, London was forced last summer to raise its level of defense against possible terrorist threats to "severe", as evidence was gathered that indicated a high probability of a possible terrorist attack.
"Outside of Iraq and Syria, we believe there have been more than 2013 terrorist plots since October 20, either directly or instigated by extremist groups in Syria," Parker said, recalling attacks in Canada, Australia, Belgium and other countries.
He warned that al-Qaeda operatives were still trying to organize large-scale attacks, and that this should be monitored and prevented in every way, even though MI5 mainly deals with smaller-scale attacks and the so-called "lone wolf" attacks which have been in vogue recently.
Parker also stated that intelligence agencies rely on the interception of e-mails and telephone conversations in order to monitor and prevent possible conspiracies, but that modern advanced coding techniques have made their task much more difficult.
British officials announced today that border procedures at ports and railway stations in Britain have been tightened after yesterday's attack in Paris.
And Spain has also announced measures to improve the protection of sensitive installations on its territory.
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