The report notes such unsophisticated but "high-impact" assaults took place recently in Edmonton and in New York.

A British woman was stabbed to death on Wednesday at a backpackers' hostel in Australia and two people were injured, one seriously, in what police say may have been an "extremist" attack.

A suspected extremist being held at a police station in western Kenya grabbed a gun and shot at police officers, killing four, leading to a hostage situation.