The flag shown being held by hostages against the window of Lindt Chocolat Cafe is not an Islamic State flag, but is an Islamic flag that has been co-opted by jihadist groups.

However, Greg Barton, a terrorism expert from Monash University, said that didn't mean there was not an Islamic State link.

He said that "getting hold of an (Islamic State) flag would be quite difficult, and people will make do with what they have got".

The flag appears to be a Shahada flag, which represents a general expression of faith in Islam, but has been co-opted by various jihadist groups.

That means it doesn't help confirm or rule out that the hostage-takers' affiliation is with Islamic State or any other group.

The translation of the flag is: "There is no god but Allah; Muhammad is the Messenger of Allah."

The Shahada is the testament of Islamic faith, which is one of the five pillars of Islam.

Monash University senior politics lecturer Ben MacQueen, whose work focuses on the Middle East, said the Shahadah - which is what the white writing on the flag appeared to be - was the testimony or core statement of belief in Islam.

Dr MacQueen said the flag looked "like the black banner which was used in the 1990s - it is not the Islamic State flag per se - it has been used in recent times, for example, by Chechen groups."

"The black symbolises the second caliphate - it is to say we [IS] are re-creating the caliphate," he said.

"A lot of Islamic States have used the black. In Afghanistan at the turn of the 20th century for example, they had just a black flag."

Dr MacQueen said the people holding hostages "may be young guys, maybe not savvy, they've got their hands on a bit of paraphernalia to suggest they have aligned themselves with the IS."

"If this was centrally organised from Syria or Iraq they would not be using that flag."

More to come...

SMH