Australian police have carried out major anti-terror raids triggered by a call from a senior Islamic State militant for "demonstration killings", Prime Minister Tony Abbott says.

The raids, involving at least 800 heavily-armed officers, took place in Brisbane and Sydney.

Fifteen people were arrested and one person has been charged with conspiracy to prepare a terrorist attack.

Police say the anti-terror operation was the largest in Australian history.

Reports in the Australian media said a plot involved beheading a random member of the public after draping them in an Islamic State flag.

Addressing a press conference, Mr Abbott said "direct exhortations were coming from an Australian who is apparently quite senior in ISIL (Islamic State) to networks of support back in Australia to conduct demonstration killings here in this country".

The news of an alleged plot to publicly behead a random Australian will shock many people here, including the vast majority of this country's long-established moderate Muslim community.

Many Muslims are unhappy with what's going on in Iraq and Syria but would never resort to violence. These raids risk antagonising the broader Islamic community.

But Australia, like many countries including Britain, is worried about the threat from Islamic State, not just abroad but at home. More than a decade on from Australia's support for the US-led war in Iraq, the country finds itself embroiled in a conflict that is far from over.

Tony Abbott this week announced he is sending 600 troops to the Middle East to assist in the fight against Islamic State. Security forces clearly face a battle at home too.

"So this is not just suspicion, this is intent and that's why the police and security agencies decided to act in the way they have."

Mr Abbott did not name the Australian concerned. Police last week issued an arrest warrant for former bouncer Mohammad Ali Baryalei, described by local media as Australia's most senior IS member.

In recent weeks, IS has released video footage showing three foreign nationals seized in Syria being beheaded.

(BBC)