Police have disrupted a terror plot to bring down a plane and arrested four men after raiding several properties in Sydney , Australia's prime minister has said.

Malcolm Turnbull told a news conference said the plot appeared to be "elaborate" rather than designed by a lone wolf.

He said: "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.

"The threat of terrorism is very real. The efforts overnight have been very effective but there's more work to do."

He said security had been increased at all major international and domestic airports around Australia. Officials did not specify if the plot targeted a domestic or international flight, but Sydney's Daily Telegraph reported that "a local route" had been the objective.


Image: Police carried out a series of raids across Sydney

The four men held after a series of raids across Sydney on Saturday were allegedly linked to an "Islamic-inspired" plot, Australian Federal Police Commissioner Andrew Colvin said.

"In recent days, law enforcement has 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."

Local media reported that 40 riot squad officers wearing gas masks stormed a Sydney house before an explosives team found a suspicious device.

Image: Police made four arrests during the raids

Mr Colvin added that several items "of great interest to police" had been seized in the raids, but the investigation was expected to be "long and protracted".

Travellers have been advised to arrive at Australian airports earlier than usual - two hours before departure - to allow for extra security screening.

The national terror alert remains unchanged at "probable".

Canberra lifted the alert level in September 2014 and introduced new national security laws amid rising concerns over attacks by individuals inspired by organisations such as Islamic State.

Since that time 70 suspects have been charged in 31 counter-terrorism police operations, according to the justice ministry.