US officials say they expect that the push will bring out more proof of the jihadists' anti-aircraft abilities, with potentially serious consequences for how the Iraqis and their coalition partners wage their war. "Based on past conflicts, [the missiles] are game changers out there," said one senior US military official, who spoke on condition of anonymity. The proliferation of anti-aircraft weaponry has also heightened concerns about the vulnerability of Iraq's airports, particularly Baghdad International Airport, the country's most important transportation hub and a lifeline for military supplies and reinforcements to Iraq. Signalling its intent to challenge US supremacy in the skies, the Islamic State recently published an online guide describing how to use shoulder-fired missiles to shoot down an Apache attack helicopter, one of the most fearsome weapons in the US Army's conventional arsenal. "Choosing the launching spot: Preferably somewhere high," the guide says in Arabic. "The roof of a building or a hill with a solid surface to prevent the appearance of dust following launching."

The authors urged "strong confidence in God and composure", and certainty "that this operation will cause a disaster to the foes and destroy their arrogance". The US has stationed about six Apaches at Baghdad International Airport. However, they have been used rarely in the aerial campaign against the Islamic State, in part because of worries about their vulnerability to ground fire and a lack of US search and rescue teams in Iraq that could respond to downed crews. The concerns also reflect the White House's insistence on limiting the number of American troops in Iraq and their exposure to hostile fire. This month, Apaches entered the battle for the first time, in co-ordination with US Air Force jets, to carry out four air strikes on a large Islamic State force north-east of Fallujah, in the sprawling desert and agricultural province of Anbar. The militants have established several strongholds there, and have continued to gain ground against Iraq's security forces in recent weeks.

Now, the Iraqi military is beginning to mount larger and more complex efforts around the country to retake territory from the Islamic State, including a counteroffensive that began more than a week ago to break the militants' stranglehold on a key refinery in Baiji, north of Baghdad. The new phase will mean an increase in the frequency of combat missions by coalition aircraft, and will likely demand a greater use of lower-flying US attack helicopters and gunships, which have important advantages in urban warfare. Since much of the most difficult fighting in the coming months is expected to unfold in the towns and cities of Anbar, US generals may be inclined to order more Apaches to support Iraqi ground troops. They may also make greater use of AC-130 gunships, a lumbering, propeller-driven plane bristling with cannons that circles at altitudes at the outer limits of some shoulder-fired missiles. As Iraqi and US officials weigh the added risk to their aircrews and, potentially, to civilian aircraft, they are particularly concerned about the threat of shoulder-fired heat-seeking missiles, commonly known as Manpads, short for Man-Portable Air Defence Systems. Syrian rebels have amassed multiple Manpad models since 2012, and the Islamic State has, generally, had little trouble acquiring any weapon used by Syrian rebels either through purchase or capture, military analysts say.

While the Pentagon's Central Command acknowledged this concern, it said it had no conclusive evidence yet that the Islamic State had such weapons. The maximum ranges and altitudes of Manpads vary from system to system but they are, generally, used against low-flying aircraft, such as fixed-wing aircraft soon after take-off or shortly before landing, or helicopters. Sunni militants in Iraq long maintained a limited, ageing stock of SA-7 Manpads, a ubiquitous Soviet-designed system that they periodically used during the American occupation from 2003 to 2011, said Charles Lister, a visiting fellow at the Brookings Doha Centre in Qatar. However, since at late 2013, the Islamic State forces in Iraq appear to have acquired more sophisticated anti-aircraft missile systems, including the Chinese-made FN-6, originally provided by Qatar and possibly also Saudi Arabia to Syrian rebels. In the images purporting to show the shooting down of the Iraqi attack helicopter, on October 3 in Baiji, the militant, a scarf wrapped around his face, was wielding a Chinese-made FN-6 missile system; apparently the first documented use of the weapon by Islamic State jihadists in Iraq, analysts said.

The militants claimed to have shot down several other Iraqi military helicopters this year, most recently a Bell 407 on a surveillance mission near Baiji on October 8. "Judging by reports from Iraq, and, in particular, Anbar province, over the past three to four months, it would seem IS have been using Manpads far more frequently and more successfully than Syrian rebels have ever done," Mr Lister said. An even greater potential concern was that militants might get their hands on SA-24s, a more sophisticated system that Russia recently sold to Iraq, and first showed up in militant videos in September, said Matthew Schroeder, a missile proliferation analyst at Small Arms Survey, an independent research project based in Geneva. He said the SA-24s had a longer range than older models and use faster and more manoeuvrable missiles. Newer systems also have a greater ability to hit targets from a wider range of angles, such as a perpendicular shot at a moving target like a plane on its approach to a runway.

As Iraqi and US officials have weighed the threats to their military aircraft, they have also taken steps to safeguard the nation's airports. The protection of Baghdad International Airport, on the western edge of the capital, was of special concern, especially since the early summer when the Islamic State's advances in Anbar and on the western fringes of greater Baghdad brought them to within 24 kilometres of the airport. New York Times