Australian plane plot may have involved bomb or gas

Four men accused of plotting to bring down a plane planned to use poisonous gas or a crude bomb disguised as a meat mincer, reports said on July 31, with Australian officials calling preparations "advanced."

The men -- reportedly two Lebanese-Australian fathers and their sons -- were arrested in raids across Sydney on July 29 evening.

The Sydney Daily Telegraph said they allegedly planned to carry the device on board a commercial flight from Sydney to a Middle East destination as hand luggage.

It said the idea was to use wood scrapings and explosive material inside a piece of kitchen equipment such as a mincing machine.

The Sydney Morning Herald also reported that a mincer was being examined, while The Australian newspaper cited multiple sources as saying it was a "non-traditional" device that could have emitted a toxic sulphur-based gas.

This, it said, would have killed or immobilized everyone on the aircraft.

Prime Minister Malcolm Turnbull said the plans were "advanced" but refused to comment on the conflicting claims over the method of attack.

"I have to respect the integrity of the investigations," he said.

"But I can say that certainly the police will allege they had the intent and were developing the capability.

"There will obviously be more to say over coming days. It will be alleged that this was an Islamist, extremist terrorist motivation."