A terrorist plot to bring down a plane was 'fairly well along' when Australian authorities moved in, US officials say, amid claims the four arrested over the plan had links to IS.
The men, named in media reports as fathers and sons Khaled and Mahmoud Khayat and Khaled and Abdul Merhi, remain behind bars and are yet to be formally charged after being arrested on Saturday.
Khaled Khayat's brother is believed to be a senior IS figure, while the other two men are related to Ahmed Merhi, who travelled to Syria in 2014, the ABC reported on Monday.
Australian authorities have refused to elaborate on the details of the plan to "bring down" a plane and won't confirm if a series of raids in Surry Hills, Lakemba, Wiley Park and Punchbowl were triggered by overseas intelligence alerts.
Now Reuters has cited two US officials familiar with the arrests as saying the Australian investigation wasn't a sting operation but the result of the detection of a developing plot.
One said it was "fairly well along" when Australian authorities disrupted it.
The target, the other official said, appeared to have been a commercial flight from Sydney to the Gulf.
Two other US officials, also speaking on condition of anonymity, said communications between the alleged plotters in Sydney and members of IS in Syria were intercepted by a foreign intelligence service.
The US officials declined to identify the service and UK officials refused to confirm or deny playing a role in detecting the plot.
Travellers at Australian airports will on Tuesday face another day of delays amid heightened security screenings.
Australian National University criminologist Dr Clarke Jones said the federal government needed to go "back to basics" and invest in prevention measures.
"It's been full steam ahead in relation to security, legislation, police and intelligence, all at the expense of community resilience and building up protective mechanisms within vulnerable communities," he told AAP.
The men are being held in Sydney under counter terrorism legislation that grants authorities the power to keep people in custody as evidence is gathered to support any charges.