AN AUSTRALIAN C-130 Hercules transport aircraft came under fire from Islamic militants during a humanitarian mission over Iraq.

The plane was attacked by small arms fire during a mission to Amerli in northern Iraq.

“It was the very first flight and the Americans flying top cover reported that small arms fire came up at the plane after it completed its pass,” a source close to the US-led operation said today.

“The fire was not observed by the Australian crew and they were unaware of it until they were informed later by the Americans.”

Prime Minister Tony Abbott and Defence chief Air Chief Marshal Mark Binskin both today said that enemy fire was not unusual in a combat zone.

“Obviously flying into a war zone ... even humanitarian airdrops into combat zones are full of risks,” Mr Abbott said.

“But the risks are reasonable given the importance of the mission they’re flying.”

The initial humanitarian mission on August 13 delivered 10 bundles of critical supplies to people trapped on Mount Sinjar by encircling ISIL forces.

After its low-level run below 300 metres to drop the vital supplies enemy fighters opened up with machine guns and assault rifles on the noisy aircraft that was completely blacked out for the night mission.

Any mission into the rebel area of Iraq is fraught with risk and defence planners are working overtime to mitigate the dangers to RAAF aircraft and aircrew.

American J-Star spy planes fly high above each flight and US fast jets are ready to respond to any ground threat.

According to sources the fighter jets did respond to the attack against the Australian Hercules with maximum force.

Defence sources today confirmed the attack took place early last month, on August 13.

News Corp Australia can reveal the aircraft was lucky not to be hit when fighters from the Islamic State of Iraq and Syria (ISIS) attacked it — as it swooped in low to drop aid packages to civilians in the town of Amirli in northern Iraq — with heavy machine guns and small arms.

The drop took place at night so the rebels would have been firing by noise rather than sight as the darkened planes flew below 300 metres to make the delivery of 15 bundles of water, biscuits and hygiene packs.

US fighter jets flying top cover for the mission responded to the enemy attack with maximum force after being guided to the ISIS fighters by a high flying US Air Force J-Star spy plane fitted with powerful ground attack radars.

“The top cover provided by American aircraft was very effective,” a source close to the operation told News Corp Australia.

Tony Abbott this morning said he had “no advice” that the Australian C-130 came under direct fire, but admitted it was entering an “active combat zone”.

“So there may well have been fire in the area,” he told 2GB Radio.

The Prime Minister continued to label the Islamic State a “death cult”, saying unlike the actions of the Nazis or Communists during the past century ISIL advertise their brutality on the internet.

“That’s why I think it’s quite proper to respond with extreme force against people like this,” Mr Abbott said.

He argued the world should be “grateful” to President Obama for his response to the situation in Iraq.

“He hasn’t been trigger happy,” Mr Abbott said.

“He hasn’t rushed in. He has been very careful about this and I think that’s to his credit.

“The last thing anyone of us should want to do is rush in to a difficult conflict in the Middle East.

“By the same token none of us should want to stand by while an avoidable, preventable genocide takes place.

“That’s why I think President Obama’s actions have been wise and just.”

Bill Shorten told his party room that “Labor would not support formed up units on the ground going to Iraq”.

“IS has unleashed a sore of hatred, violence and ignorance,” the Opposition Leader said.

He called for the issue to be judged through a “clear set of principles”.

His Deputy Tanya Plibersek described the situation as a “humanitarian disaster on an intimidatingly large scale”.

The biggest risk to Australian aircraft comes from man portable shoulder fired missiles, but a lucky shot from a rifle could potentially bring down an aircraft. Military aircraft are fitted with effective missile counter measures.

A huge Australian C-17 jet transport plane is expected to land in the nearby Kurdish city of Erbil within 24 hours carrying the first load of weapons for Peshmerga fighters who are holding the ISIS militants at bay.

The four-engine transport aircraft is a much bigger target than a Hercules and it will also be provided with top cover by the J-Star and fast jets.

It will also carry a detachment of elite RAAF Airfield Defence Guards (ADGs) from the Amberley base near Brisbane supported by a small contingent of SAS troops.

Neither the ADGs or SAS troops are defined as ‘combat troops’ or ‘ground forces’.

The attack on the C-130 highlights the high risk faced by Australian aircrews and troops undertaking the humanitarian and weapon supply missions in support of thousands of civilians under siege by the brutal ISIS rebels.

Those missions are likely to be stepped up in coming days to support a counter offensive by Iraqi and Peshmerga forces following the swearing in of a new government in Baghdad on 10 September.

The RAAF has about 400 ADGs based in Number One and Number Two Airfield Defence Squadrons with a mixture of full-time and high-readiness reserve personnel.

Unlike regular troops, the ADG mission is to defend rather than to take ground.

That means operating in small teams of between four and 10 with similar tactics to special-forces units.

- with Jennifer Rajca