Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

MELBOURNE — Australian police on Thursday charged two men with planning a terrorist act, over their role in a foiled "Islamic-inspired" plot to bring down an airplane.

The men were among four arrested last weekend in counter-terror raids across Australia's biggest city of Sydney.

Police man a checkpoint in the Sydney inner suburb of Surry Hills on Friday. William West / AFP - Getty Images

The plot spurred Australia's intelligence agency to raise the aviation threat level to "probable," prompting tighter airport security measures, before the risk was downgraded to "possible" on Thursday.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Both men have been charged with two counts each of "acts done in preparation for, or planning, a terrorist act," the Australian Federal Police said in a statement.

Police did not release details of the plot, but will hold a news conference on Friday.

The target appeared to have been a commercial flight from Sydney to the Persian Gulf, a U.S. official familiar with the arrests has previously told Reuters.

The plot may have involved a bomb or poisonous gas, domestic media have said.

Police had earlier released one of the four arrested men, but the other remains in detention without charge, under special counter-terror laws.

Abu Dhabi's Etihad Airways has said it is assisting Australian federal police in the investigation.

Since 2014, Australia has been on heightened alert for attacks by home-grown militants returning from fighting in the Middle East, or their supporters.

Although the country has suffered few domestic attacks, authorities say 13 significant plots have been foiled in that time.

The 2014 Lindt cafe siege in Sydney, in which the hostage-taker and two people were killed, was Australia's most deadly violence inspired by Islamic State militants.