Australia, U.S. Appalled at Decapitated Head Photo

A shocking image of what is believed to be the young son of an Australian man holding a decapitated head in Syria shows how barbaric the Islamic State "terrorist army" is, Prime Minister Tony Abbott said Monday.

He made the comment while announcing Australia will likely join airdrops of supplies to Iraqi civilians besieged by jihadist IS militants on a barren mountain.

The picture, taken in the northern Syrian city of Raqa, was posted on the Twitter account of Khaled Sharrouf, an Australian who fled to Syria last year and is now an IS fighter, The Australian newspaper said.

It reportedly shows Sharrouf's seven-year-old, Sydney-raised son dressed like any other young boy in blue checked trousers, a blue shirt and baseball cap, struggling to hold up the severed head of a slain Syrian soldier by his hair.

It was captioned with the words "That's my boy".

Another photo published by the newspaper shows Sharrouf dressed in camouflage fatigues posing with three young boys whom it said are believed by security agencies to be his sons.

All are holding guns in front of the flag of the Islamic State militants who have swept across Iraq and Syria, seizing swathes of territory.

Abbott, speaking to ABC radio from the Netherlands, said the pictures showed the barbaric nature of the Sunni extremists formerly known as the Islamic State of Iraq and the Levant (ISIL).

"What we've got to appreciate is that Islamic State -- as they're now calling themselves -- is not just a terrorist group, it's a terrorist army and they're seeking not just a terrorist enclave but effectively a terrorist state, a terrorist nation," he said.

"And this does pose extraordinary problems... not just for the people of the Middle East but for the wider world.

"And we see more and more evidence of just how barbaric this particular entity is."

U.S. Secretary of Defense Chuck Hagel was equally outraged, saying the militants were "a threat to the civilized world".

"I think this was reflected in a local newspaper I saw this morning and the picture on the front page. It was pretty graphic evidence of the real threat that IS represents," he said in Sydney.

Australia has an arrest warrant out for Sharrouf, who has also been pictured posing with severed heads. Officials have said up to 150 Australians are fighting alongside militants overseas, mostly in Iraq and Syria.

Sharrouf, who served almost four years in prison after pleading guilty over a 2005 conspiracy to attack Sydney, fled the country using his brother's passport.

Australian Defense Minister David Johnston said he was "revolted" by the image, which he called "a shocking misrepresentation of Islam".

"I'm very upset about this sort of thing completely coloring our view of Muslims," he said, while Lebanese Muslim Association president Samier Dandan distanced the Australian Muslim community from it.

"I stand very far from that concept -- this is an act of a lunatic," he told the ABC.

Abbott, meanwhile, said Australia was ready to take part in American airdrops to civilians threatened in Iraq, and could also deploy two aircraft for any airlift mission.

"Australia will gladly join the humanitarian airlift to the people stranded on Mount Sinjar. This is a potential humanitarian catastrophe -- President Obama has said it's a potential genocide," he said.

"So we do have some Hercules C1-30 aircraft in the Middle East and we have a C-17 that's bringing humanitarian supplies from Australia in the next day or so, and we'd expect to join that humanitarian airlift should it be needed sometime later in the week."

As well as dropping supplies, American jets and drones have been carrying out attacks on IS militants in northern Iraq as Washington tries to turn the tide on two months of jihadist expansion in the region.