We don't know the exact details of why Abbas Khan, the British surgeon who died while in custody, was arrested at a regime checkpoint in Syria and imprisoned and tortured, or the manner of his death.

However, his death comes against the backdrop of the systematic and sustained war on doctors who serve civilians in Syrian opposition-held areas.

Doing one's job, let alone responding to the humanitarian imperative, is effectively criminalised in Syria through an anti-terrorism law introduced last year. And it contravenes the international laws of war designed to protect doctors.

Khan's own reports in his letters, which describe the degrading treatment he received and hideous torture, are consistent with the accounts of other doctors, paramedics, and relief workers detained and tortured at that particular detention centre.

Of the more than 160 doctors killed I've counted since the conflict began, more than 90 have been assassinated for doing their jobs; others have been placed on "wanted" lists and at least 36 paramedics have also been killed by Syrian military snipers or shot dead at checkpoints. The experience of doctors can be summed up by the orthopaedic surgeon from the north-eastern town of Aleppo. He described to me treating a man who had been wounded by a government sniper when a military intelligence officer interrupted him, violently wrenched him away, and accused him of terrorism.

He had said to the officer: "How am I a terrorist? I am a doctor. This is my job," to which the officer replied: "We are shooting at them to kill them, don't you understand?… if you treat them, you are the enemy."

The repercussions included having his clinic destroyed, his wife's clinic was shut down and they were forced to leave Aleppo. Many doctors, fearing detention or worse, described being forced to deny treatment to people in need. Since the beginning of the violent response to peaceful uprisings that began in March 2011, doctors have been threatened.

I interviewed a paediatric resident who has not been able to complete his residency training because the children's hospital in Aleppo was destroyed last year. Of the 5000-odd doctors who used to practise in this densely populated city, there are fewer than 300 left working on the frontline, in incredibly difficult conditions - operating without electricity or anaesthesia, for example - in Aleppo's opposition-held area.

Since Aleppo's blood bank supplying the entire northern region was bombed last July, surgeons have often donated blood themselves. The massive amount of war trauma due to the continued shelling, sniping, missiles and barrel bombs mean that the demand for the skills of orthopaedic surgeons such as Khan is escalating, even as more doctors are forced to flee, ambulances are targeted by missiles, civilians are targeted and hospitals are under aerial and ground assault. Out of 91 public hospitals, five are occupied and almost 50 destroyed. Dar Al-Shifa Hospital in Aleppo was attacked at least six times last year.

Whether doctors work in emergency rooms or war zones, the humanitarian principles of impartiality and neutrality - the professional obligation to treat those who need it most, irrespective of personal or political preference - are the same. The difference is that in war zones, doctors rely on the international legal protection afforded by medical neutrality. The humanitarian space has been not only closed but criminalised in Syria. And Khan's death is just one part of the result.

The scale of the attacks against doctors in Syria is unprecedented and are not only a part of president Assad's war plan, but a deterrent. The only reason Syria is in any shape at all today reflects the courage and dedication of doctors, paramedics, medical students, residents and support staff who are willing to continue to risk their lives in order to bring relief to those who are besieged.