Death by drone in Afghanistan and Pakistan comes out of a clear sky.

There are no hiding places for those on the ground. By the time a target has been identified and verified, the sequence has all the inevitability of the pull of gravity. A trigger is pulled, the missile is fired and the first inkling of danger comes when it slams into its mark, spewing out death and destruction. To add to the surreal nature of the attack, the pilot of the weapon is not on the battlefield eye-balling the enemy but seated safely in an air-conditioned bunker thousands of miles away in the US state of Nevada or the English county of Lincolnshire.

Inevitably, because drones are basically covert weapons based on the principle of "fire and forget", questions are being raised about the legality and morality of using armaments of this kind. The UN has pledged to investigate civilian deaths from drone strikes, especially when the victims are women and children, and a legal challenge has been brought against the British Government for its part in allegedly aiding CIA drone strikes in Pakistan.

Ben Emmerson, QC, UN Special Rapporteur on Counter-Terrorism and Human Rights, said some civilian casualties could amount to war crimes. He speculated that at least 50 civilians could have been killed in "follow-up" strikes while going to the aid of victims, and more than 20 civilians may have been killed in strikes on funerals. He said the UN would set up a special investigations unit to examine the legality of civilian deaths from drone strikes. By the end of September, British drones had fired 334 guided weapons in Afghanistan since 2006, causing four civilian deaths.

Equally, though, there are differences of approach in using a drone strike in a hostile environment. From a military point of view, the use of drones is justified in the same way an air strike can be justified. These are usually called in by ground commanders when they feel under threat and need air power to help resolve a problem on the ground – for example, if they are being ambushed or are in danger of being overwhelmed by the enemy.

"That happens all the time and there's no drama about calling up a helicopter gunship or even a fast jet," says a British infantry commander with experience in Helmand. "We all know the legal parameters and, if our guys are in danger, then air support is a legitimate option."

Drones are often on standby already, circling above the battlefield, and can be called in quickly and effectively to offer fire support or provide real-time intelligence. So far this year, in the war against terrorist groups in Afghanistan, British forces have deployed 106 strikes compared to the US total of 333.

Between them that's an average of 36 strikes per month, and US and British forces are now launching more drone operations than at any time in the 11 years of the conflict in Afghanistan.

Add the covert CIA operations against targets in Pakistan – an estimated 300 in the past eight years (the exact figure is classified) – and it is clear the drone with its Hellfire missiles has established itself as a potent asset in Nato's armoury in the fighting in Afghanistan. In the past year, drone operations have almost doubled in number from 5% to 9% of all air combat operations and, to meet the demand, the US Air Force now has 61 Reaper or Predator pilotless aircraft in Afghanistan. At the same time, 10 British Reaper drones are operated by 13 Squadron from its base at RAF Waddington in Lincolnshire, where they are as much part of the air force's order of battle as any manned strike aircraft or helicopter.

"The difference between the Afghan operation and the operations in Pakistan and elsewhere comes down to the fundamental differences between open military campaigns and covert campaigns run by the intelligence community," says Peter W Singer of the Washington-based Brookings Institution's 21st Century Defence Initiative.

"It shapes everything from the level of transparency to the command and control, to the rules of engagements, to the process and consequences if an air strike goes wrong. This is why the military side has been far less controversial, and why many have pushed for it to play a greater role as the strikes slowly morphed from isolated, covert events into a regularised air war."

The numbers killed in such actions vary from source to source as battlefield casualties are no longer tabulated in the same way they were in the Vietnam War, but run into the hundreds. Not even the White House can say with certainty how many people have been killed, how many have been legitimate targets or how many have been innocent civilians killed by mistake. Some estimates put the number of civilian fatalities in Pakistan at between 450 and 800, but intelligence is imprecise.

Those operating the drone weapons system argue that, although there has been "a small number" of incidents in which civilians have been killed, the drone is still a tolerably precise weapon and less of a blunt instrument than a piloted aircraft high above the battlefield. As far as is operationally possible, drones fly on lengthy daily patrols and – aided by accurate sensors – are guided by their crews from US and UK bases. The operating personnel also have access to a huge database of people considered legitimate target. The CIA keeps a constantly updated and revised matrix on those who can or cannot be targeted.

There are, of course, differences in the way these weapons are utilised. In battlefield operations the targets are largely known and therefore beyond question, which means the moral issues are confined to the basic ethical response to killing other human beings. What makes these weapons so questionable outside the armed forces is their use by the CIA in covert and unacknowledged operations over neighbouring Pakistan or in special forces operations when they are used to target individuals suspected of being members of terrorist groups.

Under those circumstances, it is not surprising their use in Pakistan has been carried out on a nod-and-wink basis, with neither side too particular to acknowledge their existence. From time to time Pakistani politicians complain about the use of US drones over the tribal areas of the north-west frontier, but few operations take place without the complicity of Pakistani ground commanders or agents on the ground. Those involved in defining their operation are aware there have to be variations in the approach to using these weapons otherwise they will find themselves caught in a moral maze.

The bottom line, according to the infantry commander, is that drones are not only force multipliers on the battlefield, in that they provide battle-winning weapons: they can save the lives of US and British military personnel. "They work and they work well," he argues. "Not only that but, used properly, they're legit and they're here to stay."

Pilots in Vegas bomb Afghans

Unmanned aerial vehicles (UAVs), known as drones, are controlled by "pilots" from the ground, or, increasingly, follow a pre-programmed path. They basically fall into two categories: those used for surveillance purposes and those armed with missiles and bombs.

The US Reaper and Predator drones in Afghanistan and Pakistan are controlled via satellite from Nellis and Creech US Air Force base outside Las Vegas, Nevada.

Armed drones were first used in the Balkans war; their use has dramatically escalated in Afghanistan and in the CIA's undeclared war in Pakistan.

The International Institute for Strategic Studies identifies 56 different types of UAVs used in 11 countries – the USA, the UK, France, Germany, Italy, Turkey, Russia, China, India, Iran and Israel.

The total number of drones in active service in countries where the institute can calculate actual stocks is 807, but this is a huge underestimate: no data is available for China, Turkey and Russia.