Australian authorities said they thwarted an elaborate “Islamic-inspired” terrorist plot to bring down an airplane and arrested four men during raids in four suburbs outside Sydney.

Prime Minister Malcolm Turnbull announced the developments in what he called “a major joint counterterrorism operation” during a news conference Saturday.

Investigators have released little information about the suspects or the alleged plot. Australian Federal Police Commissioner Andrew Colvin told reporters that investigators believe it was “Islamic-inspired terrorism” and involved the use of an improvised explosive device.

“At this time we don't have a great deal of information on the specific attack, the location, date or time,” Colvin said, according to Reuters. “However, we are investigating information indicating that the aviation industry was potentially a target.”

Police said investigators searched homes in the suburbs of Lakemba, Surry Hills, Wiley Park and Punchbowl late Saturday. Reuters reported that the four men, whose names have not been released, have not been charged as of Sunday. It remains unclear what charges, if any, they would face. Federal police, however, warned that the four could be detained for an extended period pending the investigation.

“The major counterterrorism operation, which has occurred overnight, is continuing,” Turnbull told reporters Saturday, adding that police have seized “a considerable amount of material” from the raids.

Video taken by Reuters shows a man wearing a white blanket over his head as police were escorting him.

The operation has led to heightened security at airports in Sydney, Melbourne, Brisbane, Canberra and other major Australian cities. Authorities have advised passengers to arrive at least two hours before their flights to allow enough time to go through security screening and to limit the number of carry-on bags and checked baggage.

“Some of these measures will be obvious to the traveling public, others will not. The increased measures will include additional checks of cabin and checked baggage,” Darren Chester, minister for infrastructure and transport, said in a statement. “While it is important the public is aware of the increased threat, Australians should not be alarmed and those traveling should go about their business with confidence.”

The terrorism threat level in the country is currently at “probable.” The National Terrorism Threat Advisory System measures the likelihood of a terrorist act on a scale of five levels, with “certain” being the highest.

The threat level was elevated to “probable,” which is in the middle of the scale, in September 2014. Since then, there have been four attacks and police have disrupted 12 terrorist plots, according to the Australian national security agencies. Almost all of these occurred in Sydney and Melbourne.

The latest alleged plot is the first one in Australia targeting an aircraft and was a “pretty big threshold moment” for extremists, Deakin University security expert Greg Barton told the Associated Press.

Police have not said what items have been confiscated or what type of IED, if any, was found during the raids. But Barton told the AP that the plotters were apparently making a peroxide-based explosive device instead of a nitrate-based one, which can be detected by airport security swab tests.

The arrests came nearly two months after the deaths of two men in Melbourne in what police have called an act of terrorism. Authorities said the gunman, Yacqub Khayre, took a woman hostage in an apartment complex and later came out of a building shooting at police, the Guardian reported.

Police fatally shot Khayre and later found another man's body. The Islamic State has asserted responsibility for the attack.