The UK use of drones to kill "high-value targets" in Afghanistan should have come as no surprise. British troops are fighting in some of the most dangerous and rugged parts of Afghanistan, and drones have great attractions. They are a relatively cheap way of killing people in areas that are otherwise largely inaccessible. The lives of British troops are not put at risk. Remote pilots carry out an apparently clinical operation, with none of the gore and messiness of military combat.

Nor is it surprising that the Ministry of Defence responds to the news with bland assurances that every effort is made to ensure that drones are used in compliance with the laws of war. There are "no reports" of civilian casualties, the ministry adds. If taken at face value, these are heartening assurances. But they also illustrate the heart of the problem: the use of killer drones is shrouded in secrecy, and the accountability mechanisms that apply to regular warfare are simply absent.

Drones lend themselves to secrecy. Used without fanfare in remote and inaccessible areas, they are invisible to all but their potential victims. The military advantages are obvious, but so too are the potential rule-of-law problems. Unless governments voluntarily disclose information, human rights monitors and independent journalists are unable to verify claims that there are limited or no civilian casualties, let alone to weigh them against credible reports that hundreds of innocents have died.

That is the situation in the border region of Afghanistan and Pakistan, where the CIA is operating a secret drone killing programme about which we have been particularly critical because the US refuses to disclose the programme's legal justification, the safeguards designed to minimise civilian harm, or the follow-up inquiries conducted.

In relation to the UK, it's true at one level, as the MoD says, that using drones to kill is no different from a pilot dropping a bomb from a fighter jet or a soldier firing a gun. In each case, the legal question is whether the human beings who authorise the use of the weapon, and those who fire it, have abided by the laws of war. Drones may only be used to kill in an armed conflict. The killing must fulfil a military need, and no alternative should be reasonably possible.

In Afghanistan, where British forces are fighting armed groups and not the troops of another country, the target must have a direct connection to the combat, either as a Taliban or al-Qaida "fighter", or as a civilian who is "directly participating in hostilities". The use of force must be proportionate, meaning that commanders must weigh any expected military advantage against possible harm to civilians. Violation of these requirements could result in a war crime.

But making the case for formal legality is only the beginning. Accountability is an independent requirement of international law. When complete secrecy prevails, it is negated. Secrecy also provides incentives to push the margins in problematic ways.

Two examples will suffice. First, the US, with reported Nato agreement, has already added Taliban-supporting drug traffickers – alleged criminals – to its kill list. Second, in the wake of the December suicide bombing of CIA operatives in Khost, American drone killings have surged dramatically. In a zone of secrecy, there is no way to know if the 90 people reportedly killed in 11 subsequent strikes were legitimate targets or simply retaliatory killings.

Equally discomfiting is the "PlayStation mentality" that surrounds drone killings. Young military personnel raised on a diet of video games now kill real people remotely using joysticks. Far removed from the human consequences of their actions, how will this generation of fighters value the right to life? How will commanders and policymakers keep themselves immune from the deceptively antiseptic nature of drone killings? Will killing be a more attractive option than capture? Will the standards for intelligence-gathering to justify a killing slip? Will the number of acceptable "collateral" civilian deaths increase?

History contains numerous examples of government secrecy breeding abuse. Drone programmes are perfect candidates. In the wake of the Guardian's revelations, the onus is on the MoD to establish accountability mechanisms to show that drone killings are in fact being carried out in accordance with accepted international legal standards.