The British Prime Minister David Cameron has tied himself up in knots trying to justify the killing of three men with links to the extremist armed group Islamic State, also known as ISIS, in an aerial drone strike by UK forces in August. Cameron announced that one of the dead, Reyaad Khan, had been deliberately targeted and suggested the UK may carry out similar targeted killings in other parts of the world.

International law allows states limited authority to kill. During an armed conflict, parties are permitted to target enemy combatants. But while the UK is involved in an armed conflict against ISIS in Iraq, Cameron has not claimed that the UK is now at war with ISIS in neighboring Syria – where the killing of Reyaad Khan and his associates took place.

So, could the UK legally target someone who may be directing terrorist attacks outside of an armed conflict? International human rights law has strict rules on the intentional use of lethal force by state security forces. These say that such force can only be used where “strictly unavoidable to protect life” and where the threat to life is “imminent.”

In justifying the drone strike, Cameron told Parliament that Reyaad Khan was killed because he was recruiting ISIS sympathizers and “directing” planned terrorist attacks in the UK. The prime minister did not produce any evidence to show that the requirements for use of lethal force were met. He then claimed the attack was justified by the right to self-defense under the UN Charter. The use of force may be a lawful act of self-defense in response to an imminent threat of armed attack, but any UK operation would still need to satisfy the applicable requirements of the laws of armed conflict and international human rights law.

The UK’s legal obfuscation is especially worrying given its abysmal record on investigating, let alone prosecuting, senior members of the armed forces, intelligence officers, or politicians for alleged war crimes overseas such as in Iraq. The current government even wants to make its armed forces overseas exempt from national human rights law.

Cameron asserted that his attorney general said the killings were legal, but is refusing to publish the advice. Why? He should publish not just his legal justifications but also as much of the factual basis for the attack as possible, explaining why the threat to life was imminent and why the killings were unavoidable. After all, if the government is so sure these targeted killings were legal, it should have nothing to hide.