Police help screen passengers at Sydney Airport on July 30, 2017.

:: Security remains heightened in airports around Australia with more intense screening of luggage after law enforcement officials said they had thwarted a plan to bring down an airliner.

Prime Minister Malcolm Turnbull and Border Protection Minister Peter Dutton on Monday declined to comment on newspaper reports that Islamist extremists planned to kill the occupants of a plane with poison gas and that a homemade bomb was to be disguised as a kitchen mincer.

Four men arrested in raids in Sydney late Saturday – two Lebanese-Australian fathers and their two sons – had yet to be charged.

The government will not comment on media reports that the suspects were not previously known to Australian security officials and that their arrests followed a tip from a foreign intelligence agency.