Melbourne: Australian authorities have foiled an alleged Islamist-inspired "terrorist plot" to bring down an aeroplane with an improvised explosive and arrested four people in a series of anti-terror raids in Sydney. Australian Prime Minister Malcolm Turnbull said security has been beefed up at all major domestic and international airports following the arrest of four men.

Melbourne: Australian authorities have foiled an alleged Islamist-inspired "terrorist plot" to bring down an aeroplane with an improvised explosive and arrested four people in a series of anti-terror raids in Sydney. Australian Prime Minister Malcolm Turnbull said security has been beefed up at all major domestic and international airports following the arrest of four men.

"I can report last night that there has been a major joint counter-terrorism operation to disrupt a terrorist plot to bring down an airplane," he said. "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," he added.

Earlier, four men were arrested during anti-terror raids carried out by federal police, New South Wales police and ASIO at five properties in Sydney yesterday. 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 yesterday. Police alleged that the arrested men were planning to use an "improvised device" to carry out an attack to bring down an 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.

"We do believe it is Islamic-inspired terrorism. Exactly what is behind this is something that we will need to investigate fully," Colvin said. "We are investigating information indicating the aviation industry was potentially a target of that attack," he added. Prime Minister Turnbull 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, he 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. "Every day, every hour, we are focused on ensuring that our defences against terrorism are stronger than ever," Turnbull said.

"That our co-operation is tighter than ever. That our co-ordination is swifter than ever before. Now, we have strong transport, security systems in place in Australia, to prevent acts of terrorism,” 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.

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