Iran is shifting the focus of its attacks in the United Kingdom to harder targets, such as military bases, and targeting Jewish communities, according to the head of Britain's spy agency MI5. 

Sir Ken McCallum provided details of the growing challenges facing the U.K. in his annual threat update at MI5 headquarters in London. According to McCallum, Iran has shown to be "keen on subcontracting its U.K. violence … using criminal proxies to try to intimidate oppositionists, Iranian media organizations, and Israeli sites," CBS News reported. 

McCallum said that much of this aggression is aimed at British Jewish communities. 

Iran is also shifting to harder targets, such as military sites, he said. These activities have led to more than 50 arrests linked to Iran over the past year alone, according to McCallum.