It’s been called the biggest open secret in US foreign policy.

The use of unmanned aerial vehicles or drones to hit militants in Pakistan and Yemen has been widely reported for years, but the American authorities have generally refused to comment – until now.

Drone strikes against targets outside warzones like Afghanistan have claimed thousands of lives since 2002 and led to accusations that the US is acting illegally.

Now President Barack Obama has announced he’s rewriting the rules of engagement.

What’s new?

Mr Obama defended the use of drones as a crucial part of the fight against terrorism, but announced new policy guidelines on when they will be used.

The US will not strike if a target can be captured, a strike can be launched only against a target posing an “imminent” threat, and the government would prefer its military top brass to control the drone programme.

But the CIA will continue to control drone strikes in Pakistan, and operations will largely remain secret, with Congress given classified briefings on strikes.

Mr Obama’s speech came on the same day that the US government admitted that four US citizens have been killed by unmanned aircraft in Pakistan and Yemen so far.

The latest victim to be named is Jude Kenan Mohammad, who died in late 2011 in Pakistan’s South Waziristan tribal area.

The US authorities say he wasn’t the target of the attack. They say the only US citizen they have set out to kill with a drone was Yemeni-American preacher Anwar al-Awlaki, whose sermons inspired Roshonara Choudhry to stab the Labour MP Stephen Timms in 2010.

The other two US citizens killed were Awlaki’s 16-year-old son Abdulrahman and Samir Khan, who produced an online al-Qaeda magazine.

How many others have died?

The lion’s share of drone operations outside warzones take place in Pakistan, 95 per cent of them in Waziristan, the mountainous ethnic Pashtun region that border Afghanistan and provide shelter for militants fighting coalition forces.

The New America Foundation think-tank puts the total death toll from missiles lauched by drones in Pakistan since 2004 at 2,003 to 3,321 people.

Britain’s Bureau of Investigative Journalism has the range slightly higher at 2,537-3,533 deaths.

Since the CIA doesn’t officially acknowledge its drone programme, both organisations keep a tally by analysing reports from various international media organisations.

Both say that, according to reports, about 20 per cent of casualties in Pakistan have been civilians over the life of the drone campaign.

That percentage appears to be declining. The New America Foundation says that under President George W Bush civilian deaths were running at about 47 per cent compared to 16 per cent under Mr Obama.

In 2012, only 2 per cent of casualties were reported to be civilians by at least two news sources, while 9 per cent of the dead were classed as “unknown”.

The number of attacks carried out in Pakistan is going down too after a high in 2010, according to the foundation.

But there were more strikes than ever in Yemen last year: 46, compared to 15 in 2011. Estimates of deaths in Yemen range from 240 to 679, with civilians accounting for 5 to 14 per cent of those casualties.

Only 55 of the dead in Pakistan – 2 per cent of all casualties – were al-Qaeda and Taliban leaders.

This suggests many of the militants being killed are middle or low-ranking fighters, contrary to Mr Obama’s statement last year that drone strikes were only used to target individuals capable of mounting an “operational plot against the United States”.

Some Pakistan officials and civil society groups have put the civilian casualty rate much higher at 75 to 90 per cent, but this has been contradicted by quotes from Pakistani security officials.

Commentators on both sides of the argument have questioned the reliability of figures quoted in media reports of drone strikes.

The US military says civilian casualties have to be weighed against the likely outcome of using other, less precise weapons – and against the lives that would be lost of al-Qaeda were allowed to operate freely.

In 2011 a senior US military source told Channel 4 News: “The claims of extensive noncombatant casualties are uncorroborated.  It’s that simple.  Credible reports of civilian deaths are taken into account, period.

“If large numbers of innocent people were being killed, the Pakistanis wouldn’t stand for it. Neither would we. That’s the reality.

“This is a weapon – fueled by good intelligence – that allows us to counter an urgent and deadly threat in otherwise inaccessible places. And it’s far more precise than conventional ground operations.

“What’s the alternative to this kind of rigour, assuming the United States and its allies are unwilling to allow al-Qaeda and its friends to plot and murder freely?”

Have drones hurt the terrorists?

Osama bin Laden wrote in a letter found in the compound in Abbottabad after his death that he was “leaning toward getting most of the brothers out” of Waziristan after years of drone strikes.

He suggests that militants ought to flee to areas where there are more trees, which he says provide some protection against the “spying aircraft”. The suggestion is that drones have almost succeeded in persuading al-Qaeda’s leaders to abandon the border region as a base.

Are they legal?

The legal case for using drones in countries with which the US is not at war was set out in a leaked document obtained by the broadcaster NBC.

The Department of Justice white paper says targeting al-Qaeda operatives in countries like Pakistan is “a lawful act of national self defense”.

It says US citizens can be targeted in certain circumstances, and adds that the host country’s sovereignty is not violated if the country gives its consent or is “unable or unwilling to suppress the threat”.

This last point is important, as Pakistan’s high court has ruled that drone strikes are illegal, and has ordered the country’s government to put pressure on the US to call a halt to them.

Previous Pakistani leaders privately gave the US the green light to hit militants with drone strikes, but the new government has pledged to end any secret deal.

International law experts and human rights groups have queued up to attack the legality of drone warfare, but the issue has never been settled by an international court or by the United Nations.

This may be a reflection of US power, or it may have something to do with the fact that as many as 80 countries are now thought to have the capacity to use drones for military purposes.

What about Britain?

The MoD revealed last month that its armed Reaper drones are now being guided from RAF Waddington in Lincolnshire. The Reaper fleet doubled from five to 10 aircraft last year.

The MoD has told the Guardian it has 500 drones in total, most of them only capable of reconnaissance, and the use of unmanned aircraft will grow over the next decade.

The official line is that drones will only be used to support troops in Afghanistan and will not be used against targets in Pakistan.

Britain’s policy is that only fully trained military pilots are allowed to fly them, whereas US drone operators get about half the training of a combat pilot.

Human rights lawyers have asked the Foreign Office to confirm or deny reports that Britain has shared intelligence with the CIA, helping it to target militants with drones in Pakistan.

By Patrick Worrall