DRIVING out terrorists in Iraq will not be a weekend campaign , Defence Secretary Michael Fallon has warned, after the first sorties by RAF warplanes over the country failed to find targets.

The Ministry of Defence confirmed the third mission to be carried out by Tornado GR4 fighter bombers since they were told to commence airstrikes had ended with them returning to their base at RAF Akrotiri in Cyprus with payloads intact.

Mr Fallon said operations could continue for weeks or even months. "This is not a weekend campaign," he said. "This is going to take a long time."

Meanwhile, David Cameron insisted there was a "comprehensive strategy" in place for defeating IS (Islamic State), which has seized large swathes of Iraq and Syria.

Speaking on BBC1's The Andrew Marr Show yesterday, Mr Cameron said: "We are part of a large international coalition to degrade and ultimately destroy this organisation. But it can't be done unless the countries where this organisation has grown up play their part in destroying it.

"But if what you are saying is that we need an uprising of the Sunni tribes, rejecting these extremists and saying 'we want to be part of a democratic pluralistic Iraq', then yes, of course we do need that.

"Our strategy here is not some simplistic 'drop a bomb from 40,000ft' and think you can solve the problem. This is one part of a comprehensive strategy to build an Iraq that has a democratic, inclusive government for everyone and, in time, Syria needs exactly the same thing."

The intervention has been backed by the family of David Haines, the Scottish aid worker kidnapped and beheaded by IS militants earlier this month, but a friend of a current British captive has voiced fears that air strikes could ruin attempts to secure his release.

Shameela Islam Zulfiqar, who travelled on aid convoys in Syria with Alan Henning before the 47-year-old was kidnapped, said: "Any air strike with UK backing is going to have a detrimental effect on the negotiations that have been going on to try to have Alan released.

"If you go by the pattern we have seen over the past four weeks ... it doesn't leave you very optimistic, but we will still have to remain hopeful.

"To give up hope completely, when we don't know, is not going to help us or the family to cope with it. It's a very difficult situation."

Along with Mr Haines, American journalists James Foley and Steven Sotloff have been beheaded in separate videos released on social media by the extremist group, while another Briton, journalist John Cantlie, has appeared in propaganda videos.

The development comes as anti-war demonstrators prepare to hold a rally in London on Saturday to protest against Parliament's decision to join the attacks on Islamic State militants in Iraq.

Groups including the Stop The War Coalition have decided to hold an event in central London on Saturday.

Lindsey German, convener of the Stop the War, said: "This war will not help make the region safe, nor will it stop terrorism across the world.

"Instead it will create new devastation and wider wars. If we want to stop IS, maybe we should start with ending the arming and funding carried out by Saudis and the tacit support given by Turkey.

"IS are the result of past interventions, all of which have failed."