A major offensive to liberate Islamic State’s last stronghold in Iraq will begin within weeks the Defence Secretary has said, as it was disclosed the militants have tried to shoot down RAF jets with surface-to-air missiles.Tornado and Typhoon jets are carrying out the RAF’s most intensive strike campaign for more than 25 years as they launch scores of attacks to back Iraqi and Kurdish forces preparing to move into Iraq’s second city of Mosul.British commanders have for the first time disclosed the risks of the air campaign as they admitted Islamic State in Iraq and Levant (Isil) fighters have repeatedly used missiles to try to bring down jets. He went on: “There is always the possibility that the piece of air that you are flying through could be contested. It might be contested by a Daesh bullet or a Daesh missile.“As the campaign changes they will try different tactics and certainly firing at coalition aircraft is a tactic which thus far has proved to be pretty fool hardy for those who are doing it.” “They are not a comfy armchair up there. Toilet arrangements are possible, but challenging.”As the jets patrol a 30 miles square ‘kill box’, every 90 minutes or so they climb to around 29,000ft to meet a Voyager refuelling tanker that accompanies them.Pilots must guide their refuelling nozzles into a waiting basket-shaped drogue suspended at the end of 90 feet of hose, while both aircraft are travelling at around 480mph and buffeted by turbulence.Once connected, the jets take on thousands of kilograms of aviation fuel, which allows them to continue their mission. There is no doubt now that Daesh is facing defeat. Indeed we are on the cusp of liberating the last major city it holds in Iraq – MosulMichael Fallon They try to shoot at our aeroplanes. When they do, they expose themselves and invariably when they expose themselves, we strike themAir Cdre Martin ‘Sammy’ Sampson Pairs of aircraft from the 903 Expeditionary Air Wing based at RAF Akrotiri in Cyprus fly armed reconnaissance missions day and night, circling above northern Iraq collecting intelligence or striking targets in sorties lasting six or seven hours.“They are long old missions to be strapped to a single seat,” explained one Typhoon pilot who cannot be identified for security reasons. This week the Telegraph joined an RAF Voyager refuelling tanker to accompany Typhoon and Tornado jets carrying out close air support sorties near Sharqat and north of Ramadi. Iraqi forces are gathering in staging areas such as Qayara air base before the assault on MosulCredit:Susannah George/AP British pilots have used evasive manoeuvres, electronic defences and decoys to avoid ground fire including missiles and anti-aircraft artillery.The militants, also known as Daesh, have launched “sporadic” attacks against RAF planes in the past 12 months.Air Cdre Martin ‘Sammy’ Sampson, commander of the UK’s air war, said: “They try to shoot at our aeroplanes. When they do, they expose themselves and invariably when they expose themselves, we strike them. Sunni militants used ageing shoulder-launched, anti-aircraft missiles against US forces throughout their occupation of Iraq. But Isil fighters and other rebel groups have since either looted or been given more sophisticated versions. Several Iraqi government and Syrian regime aircraft have been shot down.Air Cdre Sampson said he could not recall a British aircraft being damaged by ground fire and “we’ve not had anything that’s caused us any problems”.RAF aircraft have carried out more than 200 strikes to help Iraqi and Kurdish troops manoeuvring on Mosul since June. Air Cdre Martin ‘Sammy’ Sampson, UK air component commander for the war on IsilCredit: Heathcliff O’Malley Ground crew working on a Typhoon fighter bomber at RAF AkrotiriCredit:Heathcliff O’Malley An RAF Tornado GR4 refuelling over IraqCredit:Heathcliff O’Malley While their aircraft stock up on fuel, the pilots keep going on sandwiches, soft drinks and chocolate bars.Another Typhoon pilot explained: “Yesterday I was halfway through eating a sandwich when we got called in to do a second strike, so with a mouthful of beef and pickle sandwich I was trying to get on the radio.”Last month the RAF announced it had carried out more than 1,000 strikes since the air war against Isil began in September 2014.One Tornado navigator, who earlier this week helped destroy a truck bomb being ferried by barge across the Tigris, said: “You never forget the sense of danger that is involved, both for you and more importantly for those on the ground. We are there to support the Iraqi security forces and they are the ones who are down in the fight.“When a target gets hit and you know that that vehicle could have potentially been driven into a town, or anywhere where there are civilians or friendly forces, and exploded, and therefore we have denied that to the enemy, then that’s a satisfying feeling.” The crew of a Tornado GR4 aircraft at RAF Akrotiri Credit:Heathcliff O’Malley Michael Fallon said: “There is no doubt now that Daesh is facing defeat. Indeed we are on the cusp of liberating the last major city it holds in Iraq – Mosul.”The city of around 1 million residents was seized by Isil in June 2014 as the Iraqi army melted away in the face of the militants’ dash across the Syrian border.An Iraqi army plan to encircle and cut off the estimated 3,000 remaining Isil fighters in the city is scheduled to begin next month. Want the best of The Telegraph direct to your email and WhatsApp? 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