A terror 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 Islamic State.

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.

The son of one of the men arrested studied aviation management and mixes with a network of pilots and airline workers, the Fairfax Media reported.

The report also quoted unnamed sources as confirmed that the Sydney cell behind the alleged plot was talking to foreign fighters in Syria in communications that were picked up by allied intelligence agencies.

Australian authorities have refused to elaborate on the details of the plot.

Two unconfirmed theories have emerged. The perpetrators planned to either smuggle a bomb onto a plane inside a kitchen grinder, or release poisonous gas into the cabin.

Various media have quoted unnamed government sources as saying the group had already attempted to smuggle a device onto a plane in Australia.

Reuters 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.

Their claim contradicts a report by ABC’s 7.30, which on Monday night said Australian authorities raided the plotters days after receiving a tip-off from overseas and and at the prompting of UK authorities.

One of the US officials told Reuters the target of the plot appeared to have been a commercial flight from Sydney to the Arabian 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.

ABC’s 7.30 on Monday night claimed the plot was uncovered when a foreign intelligence agency intercepted communications to conspirators from Syria.

“[Authorities] have made this disruption only three days after hearing a tip-off, presumably from a partner agency overseas, that attack-planning by this cell was imminent,” Jacinta Carroll, head of the Australian Strategic Policy Institute’s counter-terrorism policy was quoyed as saying.

7.30 said police hoped to gather more evidence before arresting the men, but the British Government declared it would issue a public security alert for Australian travel if the raids did not happen promptly.

UK officials have 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.

– With agencies