Jiahdi John urges supporters to make Britain pay for killing 'brothers' in Syria

British jihadist and Islamic State's (IS) chief executioner Jihadi John, also known as Mohammed Emwazi has sent dozens of encrypted messages to the group's supporters in the United Kingdom, urging them to make Britain pay for killing "brothers" in Syria.

In one of the messages, the executioner also used the phrase "behead the infidel" in revenge for RAF strikes in Syria, reported The Daily Star.

The messages were intercepted by authorities who hacked into encrypted emails at the Government's eavesdropping base GCHQ. Spy chiefs warned that U.K.-based terror cells would attempt a "spectacular" attack in the next few months.

MI5 head Andrew Parker also briefed Prime Minister David Cameron on the "imminent threat" which prompted last week's clampdown on home-grown extremists who pose a threat to the U.K.

Parker also warned Cameron that an IS attack on a major city was "effectively inevitable".