DOCUMENTS revealed by the US National Security Agency (NSA) whistleblower Edward Snowden have raised the possibility that British intelligence may have helped guide American drone strikes on areas outside of conventional war zones.

Britain has carried out strikes by drones – also known as unmanned aerial vehicles (UAVs) or remotely piloted aircraft (RPAs) – in war zones such as Afghanistan, Iraq and Libya.

However, the papers showed how closely the NSA has worked with its British counterpart Government Communications Headquarters (GCHQ) in Pakistan and Yemen. They give details about how terrorism suspects are targeted and how the operations can go wrong.

America drone strikes – usually carried out by the CIA or the Joint Special Operations Command – came under fresh scrutiny earlier this year when President Barack Obama revealed that such an operation had killed two Western aid workers who were being held hostage in Pakistan by al-Qaeda. Intelligence officials who had been monitoring the compound did not know the hostages were there.

Last week it emerged that an Algerian terrorist who was widely reported to have been killed in a US strike appeared to be still alive. And several days after a strike in Yemen, US officials learned that it had killed a senior member of al-Qaeda’s worldwide leadership.

According to the British documents, US officials ordered a drone strike in Yemen in 2012, to kill a doctor they believed was working with the terror network to surgically implant explosives in suicide bombers.

An internal GCHQ publication identified the doctor as Khadim Usamah, who was described as “the doctor who pioneered using surgically planted explosives”. It said Usamah, who appears not to have been identified publicly before, was a member of al-Qaeda in the Arabian Peninsula, the branch based in Yemen. It added that he was killed along with a second member of the group.

GCHQ said it would not comment on intelligence operations, but added in a statement: “We expect all states concerned to act in accordance with international law and take all feasible precautions to avoid civilian casualties when conducting any form of military or counterterrorist operations.”

Both the CIA and NSA refused to comment.

There has been close co-operation between British and American security services, particularly in the signals intelligence – eavesdropping – area, and some of the GCHQ papers suggested it provided intelligence for the Yemeni, and other US strikes.

The documents spoke of GCHQ employees at the NSA station in Fort Gordon, Georgia, and at Menwith Hill, another NSA station in North Yorkshire.

CO-OPERATION in targeted killings is not a subject for public discussion by British intelligence officials and, in a written answer to a parliamentary inquiry last year on UK participation in a UAV strike in Yemen, then defence minister Mark Francois, replied: “UAV strikes against terrorist targets in Yemen are a matter for the Yemeni and US governments.”

A poll last year showed the majority of the American public (52 per cent) supported the drone strikes, while Britons largely opposed them – 59 per cent were against. Their opposition was largely down to reports of civilians being unintentionally killed.

However, while supporters said that missiles from unmanned aircraft were the most effective way to eliminate terrorists, intelligence agencies often do not have enough detailed information about who is in a strike zone to be sure that all pose a terror threat.

The GCHQ papers stress the essential role of eavesdropping and the tracking of electronic signals in identifying suspects, as well as calculating their exact location.

This intelligence is vital in improving the chances of finding and hitting the correct target.

But the same documents also indicate the incorrect conclusions that signals intelligence can reach.

A smartphone carried by a target, for example, can be easily tracked by GCHQ or the NSA and can help positively identify a target. However, the phones can be passed from person to person, leading to mistaken identifications.

A 2010 GCHQ guide to targeting said: “Of significant note is whether the handset is identified as single user or multi-user.”

IT added that because of uncertainties like that, the agencies tried to identify targets by both voice and physical appearance.

The guide said some suspects were more “comsec aware” than others, and paid more attention to communications security knowing that their calls were probably being tracked.

It also talked about a suspect “detaching” from communications – ending a call or turning off a mobile phone – and notes that someone who is talking on the phone will “detach” when hit by a missile.

“Immediately after a strike it should be possible to detect whether the target detached at time of strike. This is a good indication that the correct target has been struck,” it said.