David Cameron: Returning jihadists are 'enemies of the state' Left unchecked, the terrorist network "could become a rallying point for the alienated and disaffected in every continent, spawning imitators and spurring individuals in far-flung places to commit stupid, destructive, suicidal acts," he said. The father of a British medical student turned jihadist has said his son should himself face execution if he is proved to be one of the killers alongside the British militant 'Jihadi John' in the film released at the weekend. Ahmed Muthana, said he feared his son, Nasser Muthana, was one of a gang of camouflaged Isil fighters who lined up and beheaded at least 18 Syrian military prisoners. Officials believe other foreign fighters seen in the unsparing 15-minute-long film include up to two French jihadists and possibly militants from Australia, Germany and Denmark. The video is the most graphic yet to accompany the execution of Western hostages and features close-ups of several unmasked militants during the beheading of Syrian officers and pilots. Mr Muthana said he had seen stills of the video and feared his 20-year-old son was in it, but was unable to confirm that. He said his son should face a death sentence if proved to be a killer. Mr Muthana, 57, said: "If that's what he has done to someone then that is what should be done to him. I am like any other father. I am trying not to believe it is my boy, but it looks like him. But I am not going to make excuses for him. He is a grown man and he must face up to what he has done. Mr Muthana, a retired electrical engineer, said: "What they are doing is inhuman, this is not the son I brought up. He has been got at - he has changed. Nasser Muthana was a promising student with a university place to study medicine before he travelled to Syria last year to wage jihad against Bashar al Assad's regime. He was followed months later by his younger brother Aseel, aged 17. Nasser then appeared in a recruiting video released earlier this year urging would-be jihadists to join him fighting for Isil. Mr Muthana, who has four children, said he had not heard directly from either son since they left and the man in the film was thinner than he had last seen his son. It resembles him, he said. I was shown a picture of the video. I cannot confirm it is him, but I think it might be. French prosecutors confirmed one of their citizens was in the film and said they were investigating whether another was present. Francois Molins, a Paris prosecutor, said 22-year-old Maxime Hauchard who had been tracked by French security agencies since he left for Syria in 2013 was in the film. Mr Cameron has said he wants a raft of new anti-terror laws, including powers to strip teenage jihadists of their passports and bar airlines from landing in the UK if they fail to provide passenger information. Asked by Philip Hollobone MP if returning jihadists should be prosecuted for treason, the Prime Minister said: "I absolutely agree with my honourable friend." The fact that people who were born, brought up, and educated in our country have been radicalised in this way and are murdering other British citizens in the deserts of Syria, makes me sick to the stomach. It is absolutely appalling that this is happening. It is not only the full force of the law that these people should face; they should also recognise that when they take up arms in this way in another country, they become enemies of the state. An estimated 15,000 militants from at least 80 nations, including hundreds of Europeans, are believed to have travelled to Syria to fight Bashar al-Assad's regime. Around 500 Britons are fighting, along with around 400 French, 300 Belgians, 250 Germans, 130 Americans and 250 Australians. Theresa May, the Home Secretary, said that the UK is now facing a "more serious threat that it has in recent years" because of the rise of Isil.