SYDNEY (AFP) - An 18-year-old man was charged on Friday (Sept 9) after allegedly making threats at the Sydney Opera House, just days after the so-called Islamic State in Iraq and Syria (ISIS) urged followers to target high-profile Australian sites.

The police said the teenager was spotted acting suspiciously in the forecourt of the building on Thursday, and there were reports that he was making extremist remarks.

He was detained without incident and his backpack has been sent for forensic examination, with police stressing "there is no ongoing threat in relation to this incident".

"A search warrant was later carried out at the man's home, where a number of items were seized for examination," the police said.

"The man was interviewed and charged overnight with threatening to destroy or damage property."

He was refused bail and due to appear in a Sydney court later on Friday.

His arrest came just days after a new English-language magazine aligned with ISIS singled out prominent Australian locations at which followers should kill people.

"Kill them at the MCG (Melbourne Cricket Ground), the SCG (Sydney Cricket Ground), the Opera House, and even in their backyards," it said.

Prime Minister Malcolm Turnbull responded by saying on Tuesday the threat of a terror attack in Australia was "real".

Officials say they have stopped 10 terror attacks in Australia in the past two years.