French prime minister Manuel Valls on Sunday condemned the execution of US aid worker Peter Kassig and Syrian soldiers as a ‘barbaric act’ which only boosted France’s determination to take on Islamic State jihadists. British PMDavid Cameron said he was ‘horrified’ after the ISIS jihadist group claimed to have executed Kassig who was also known as Abdul-Rahman after converting to Islam. He said, ‘I’m horrified by the cold blooded murder of Abdul-Rahman Kassig,’ Cameron wrote on Twitter. ‘ISIS have again shown their depravity. My thoughts are with his family.’ The ISIS video, which is being examined by US intelligence officials and Britain’s Foreign Office, also showed simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions carried out by IS militants.