Terming this a "new and worrying development" that "could get worse as events unfold," MI5's Director General Jonathan Evans said in a rare public speech here as his agency braces for an array of threats ahead of next month's summer Olympics.
Warning "the (Olympic) games present an attractive target for our enemies and they will be at the centre of the world's attention in a month or so," he said that efforts have been taken to ensure that the sports extravaganza did not become an easy target.
The MI chief said Iran was another source of potential concern, as the west and Israel put pressure on Tehran to abandon any nuclear weapon ambitions it may secretly harbour, The Guardian reported.
He said there had been a series of attempted plots against Israeli interests in India and Azerbaijan, as well as a plan by Iran's Islamic Revolutionary Guard Corps to attack the Saudi ambassador in the US.
Instability in the Arab region had created "a permissive environment for al-Qaeda", Evans said adding that there was evidence that Britain's "would be jihadis" were travelling in search of training and opportunities for terror activity.
Al-Qaeda, he said, was shifting away from Pakistan and Afghanistan towards Yemen, Somalia and the Sahel, the MI chief said.
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"We appear to be moving from a period of deep and focused threat to one where the threat is less monolithic but wider," he added.
Though Evans did not say how many people from the UK had gone to the region, it is believed the agency has monitored more than 100 who have attempted to link up with extremists in countries such as Yemen, Egypt, Syria and Libya.