AIRPORT security has been ramped up and travellers should brace for delays and extensive screening measures following overnight terror raids in Sydney.

Speaking this morning, following the arrest of four men over an alleged terror plot that reportedly involved blowing up an aircraft, Prime Minister Malcolm Turnbull said the country had strong security systems in place to prevent acts of terrorism.

The Prime Minister confirmed the investigation was ongoing and the arrests were made in connection with a bomb plot that involved an aircraft.

The government also confirmed it was an Islamic State inspired plot.

Mr Turnbull said security has been increased across all major airports as a result of the investigation. The four men have not been charged.

“Every day, every hour, we are focused on ensuring that our defences against terrorism are stronger than ever,” Mr 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.”

It is not yet clear if the alleged plot targeted a domestic or an international flight.

Mr Turnbull said measures put in place at Sydney airport on Thursday have now been extended to all major airports at domestic and international terminals around the country overnight.

“Now this ensures national consistency of our aviation security arrangements,” he said.

“Some of the measures will be obvious to the public, some will not be.

“Travellers should be prepared for additional scrutiny at screening points, and while it is important that Australians are aware of the increased threat — be assured we have the finest security and intelligence services in the world and they are working, as is my government, and all our governments around Australia, night and day, to keep Australians safe.”

The government wouldn’t elaborate on what exactly the new measures involved, however the office of transport security has been advised that security screening will now take longer.

Travellers have also been advised to arrive at terminals at least two hours before flights and to limit the amount of carry-on and checked baggage to help ensure security screening remains efficient.

Speaking about last night’s raids, Justice Minister Michael Keenan said this was the 13th significant threat since 2014 when the terrorism level was raised to probable.

“Since 2014, 70 people have been charged as a result of 31 counter-terrorism operations around the country,” he said.

“The primary threat to Australia still remains lone actors, but the events overnight remind us that there is still the ability for people to have sophisticated plots and sophisticated attacks still remain a real threat.

“In light of this information, it’s very important that everyone in Australia remains vigilant.”

‘EXPECT DELAYS’

Australian Federal Police Commissioner Andrew Colvin said officers became aware people in Sydney were planning to commit a terrorist attack using an “improvised device”.

Commissioner Colvin said police believed the attack would target the Australian aviation industry at a major airport.

“In recent days, law enforcement has been become aware of information that suggested some people in Sydney were planning to commit a terrorist attack using an improvised device,” he said.

“We are investigating information indicating the aviation industry was potentially a target of that attack.”

Commissioner Colvin said there was no evidence that airport security had been compromised and confirmed it was believed to be an ISIS inspired plot.

“We believe it’s Islamic-inspired terrorism,” he said when asked if the Islamic State group was behind the plot.

“Exactly what is behind this is something we need to investigate fully.”

NSW Police Commissioner Mick Fuller also would not be drawn into details about what the screening measures involved but said travellers should expect delays.

“There’s an increased police and security agency presence, you can expect longer delays to make sure that more screening is being done on baggage, both hold luggage as well as hand luggage, and the travelling public just need to be aware to get about their business, go to the airport, but give themselves more time,” he said.

TERROR RAIDS

Heavily armed police raided properties in four Sydney suburbs and arrested four men over the alleged terror plot.

Mr Turnbull said last night the counter-terrorism raids on Saturday afternoon were designed to stop “terrorist attacks in Australia”.

Australian Federal Police, NSW Police and the domestic spy agency ASIO jointly carried out the operation on Saturday afternoon in Surry Hills, Lakemba, Wiley Park and Punchbowl.

“Four men have been taken into custody and are assisting police with their inquiries,” the AFP and NSW Police said in a joint statement following the raids.

The raids, which had been planned for several days, targeted an alleged Islamic extremist cell and are understood to have been brought forward for operational reasons, the Herald Sun reported.

TV footage showed a man with a bandage on his head and draped in a blanket being led away by authorities.

One woman told the ABC her son and husband were arrested after her Surry Hills house was raided. She denied they had any links to terrorism.

“I love Australia,” she said.

Mr Turnbull was briefed of the operation by security agency heads.

“These operations are designed to disrupt and prevent plans to undertake terrorist attacks in Australia,” he said in a statement.

“My number one priority, and that of my government, is the safety and security of all Australians, and the public should be reassured that our security and intelligence agencies are working tirelessly to keep us safe.”

The family who lived in the raided house on Cleveland Street in Surry Hills have been described as “perfectly nice and normal people” by a neighbour.

“We knew them to say hello to and they seemed nice,” the woman in her early 30s, who didn’t want to be identified, told AAP.

An elderly couple lived in the home, the neighbour said, and they had adult children.

The neighbour came home while the raid was underway and said there were “heaps” of police at the scene.

— With AAP