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Australian Prime Minister Malcolm Turnbull said Sunday that police conducted a counterterror operation "to disrupt a plot to bring down an airplane," and announced increased security at the nation's airports.

"The operation is continuing. At this stage, four people have been arrested and a considerable amount of material has been seized by police," Turnbull said in an address at about 7:30 a.m. Sunday local time (5:30 p.m. Saturday ET).

The identities of those arrested were not immediately released, and Australian Federal Police Commissioner Andrew Colvin said they had not yet been charged.

According to Colvin, the plot included the potential use of "an improvised device" and the "aviation industry was possibly the target of the attack."

The Associated Press reported that the plotters were creating a peroxide-based explosive device that is harder to detect by airport security. Salman Abedi, the Manchester Arena bomber, used a similar type of explosive to kill 22 concert attendees in England in May.

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"We believe it's Islamic-inspired terrorism," Colvin said when asked if the ISIS group was behind the plot.

Police man a check point in the Sydney inner suburb of Surry Hills on July 29, 2017. Four people were arrested in a series of raids across Sydney on July 29 which the prime minister said were carried out to foil plans for terrorist attacks in Australia. William West / AFP - Getty Images

Australian Federal Police said the multiple searches were conducted in the Sydney suburbs of Lakemba, Surry Hills, Wiley Park and Punchbowl. The searches were expected to continue for "many hours and days," according to Colvin.

Turnbull said additional security measures were put in place at Sydney’s airport since Thursday, and those and other measures were extended to all major airports around the country overnight.

"Some of the measures will be obvious to the public, some will not be," Turnbull said. He said travelers should be prepared for additional scrutiny at screening points.

Turnbull called the arrests "an example of the outstanding work that is conducted by the joint counter terrorism team."

Australian Prime Minister Malcolm Turnbull, left, speaks during an address to the media as Australian Justice Minister Michael Keenan, right, looks on, in Sydney on July 30, 2017, following counter terror raids. Sam Mooy / EPA

Australian Federal Police and New South Wales police said in a joint statement that the four men taken into custody "are assisting police with their enquiries."

"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 to keep Australians safe," Turnbull said.

The nation’s terror threat level remains at probable, according to the country’s National Terrorism Threat Advisory System.

Australia has seen several terror attacks or planned attacks in recent years. In June a 29-year-old Somali-born Australian was shot and killed by police after killing a man and taking a woman hostage in Melbourne, in what authorities have said was an act of terrorism. Police said they believe the attacker, Yacub Khayre, acted alone.

In December police said they disrupted a plot to bomb parts of Melbourne on Christmas Day, in a planned attack that authorities said was inspired by the terror group ISIS.

Australia raised its terrorism threat level to "probable" in 2014 in response to threats posed by ISIS. Justice Minister Michael Keenan said Sunday that 70 people have been charged in counter terrorism operations since 2014.