Police allege the group of Sydney men were planning to use an “improvised device” to carry an attack to bring down aircraft but no details on a particular target, location, time or date were uncovered.

Australian Prime Minister Malcolm Turnbull on Sunday said that anti-terror raids carried out in Sydney on Saturday “disrupted” an alleged plot to bring down an aircraft, even as four people were held in this connection.

According to media reports on Saturday, four men were arrested during anti-terror raids carried out by Australian federal police, New South Wales police and ASIO at five properties in Sydney.

“We face a range of terrorist threats. Some of them are lone actors, who activate very quickly with very little warning. On other occasions you get quite elaborate conspiracies — this appears to be in that category,” Mr. Turnbull said.

Police allege the group of Sydney men were planning to use an “improvised device” to carry an attack to bring down aircraft but no details on a particular target, location, time or date were uncovered.

Australian Federal Police Commissioner Andrew Colvin said they had no reason to believe airport security had been “compromised” but a heightened state of security has been implemented.

Police said that they found the material and items that could be used to make a homemade bomb, when they raided the house at Surry Hills on Saturday.

Mr. Turnbull said the “major counter-terrorism operation” was underway and extra security measures had been put in place at major airports.

He said those with plans to travel should do so with confidence, but allow extra time for security screening because of the threat.

“Some of the measures will be obvious to the public, some will not be — those travelling should go about their business with confidence,” Mr. Turnbull said.

“Travellers should arrive at terminals at least two hours before flights to allow ample time for screening.

“They should limit the amount of carry-on and checked baggage, as this will help to ensure that security screening is efficient,” he said.

Reports said that authorities had received an information on the alleged plot on Wednesday and beefed up security at Sydney Airport on Thursday.

NSW joint counter-terrorism team decided to raid last evening following which four men were in custody at the Sydney Police Centre in Surry Hills.

The men could be held without charge for around a week.

Five properties were raided were in Surry Hills, Punchbowl, Wiley Park and Lakemba.

Meanwhile, screening at all major Australian airports has been ramped up overnight and air travellers have been warned to expect delays.

Virgin Airlines and its subsidiary Tigerair released a statement confirming the additional security measures.

“The travelling public can expect to experience an increased level of security scrutiny at the airport but they should not be concerned about these precautionary measures,” the statement read.

“As the measures place an additional burden on the screening system, it may take a little longer than usual to get through the process,” it read.