Islamic State (formerly known as Isis) jihadists have downed an Iraqi army helicopter during fighting near the northern refinery town of Baiji, days after the extremist group issued guidelines to target such aircraft.

Military aviation officials in Baghdad said the two pilots on board were killed as the Bell 407 was hit by a shoulder-fired missile.

It was the second Iraqi helicopter shot down by the group near the city home to Iraq's biggest oil refinery in less than a week, as a Mi-35 was downed by a rocket launcher on 3 October.

The attack came days after the US admitted using attack helicopters as part of an international coalition air offensive against the extremists, underscoring the risks related to the operation.

"Fixed-wing aircraft flying at 30,000ft are completely immune from the type of weapons that Islamic State fighters have, but a helicopter is not," former navy aviator Christopher Harmer, who is now an analyst at the Institute for the Study of War think-tank, said.

"When you're flying a helicopter 150ft above the ground, that helicopter can be shot with a rocket-propelled grenade or a heavy machine gun ... so yes, it is much more dangerous" .

US Defence officials said two Apache helicopters struck IS targets near northeast of Fallujah, Anbar province at the weekend.

In response, the extremist group circulated online a memo instructing jihadists on how to use those weapons against coalition aircraft.

IS militants are known to have looted portable surface-to-air missiles, such as the Russian-made SA-16 and SA-18 and the American FIM-92 Stinger, from Syrian and Iraqi army bases they took over.

The paper provided mujahideen fighters with detailed guidelines on how to effectively aim, fire and shoot against a helicopter.

Suggestions include choosing an elevated firing spot, such as the top of a hill or a building rooftop, and surprising the helicopter crew shooting when they are busy firing at other targets.