Australian authorities increased security checks at all major airports in the country after police uncovered a terror plot to bring down an aeroplane, The Sydney Morning Herald said on Sunday. Flyers have been warned of delays.

The police raided five properties in Sydney’s Surry Hills, Punchbowl, Wiley Park and Lakemba on Saturday evening and took four people into custody. The arrested men were planning to use an “improvised device” to target the aviation industry, police said. However, no details of a place, time or target have been revealed yet.

Australian Prime Minister Malcolm Turnbull said the threat of terrorism is “very real” and that airport security has been “enhanced and intensified” on account of Saturday evening raids.