Hostages seen through the window of the cafe in Sydney.Twitter/Stephen Wilson

The hostage crisis in a Sydney cafe, in which a gunman has taken several people hostage in the Lindt Chocolate Cafe in Martin Place, has sparked fears of a terror attack after an Islamic flag was seen on the window.

In September, Islamic State (IS) spokesman, Abu Muhammad al-'Adnani, had told Muslims in Australia to carry out lone-wolf attacks against civilians, SITE Intelligence had reported.

The incident has elicited reactions by pro-ISIS supporters on Twitter. One account 'Milk Shami', which claims to be 'pro-caliphate' and 'loves Islamic State' has tweeted - "I love this Lindt Chocolate" and "Let the flag guessing game begin". 

An Islamic flag seen on the window has triggered fears of a terror-driven hostage crisis, though authorities have not labelled the incident as a 'terror-related event'.

Terrorism experts however believe that the flag could be symbolic of the terror group.

"Getting hold of an [Islamic State] flag would be quite difficult, and people will make do with what they have got," Greg Barton, a terrorism expert from Monash University, told The Syndey Morning Herald.

However, pro-jihad posts on Twitter have added another dimension to the crisis, by asking for a 'guessing game' about the flag, while some jihadists have put the blame on Hezbollah. 

Some tweets are mentioned below - 

"Let the flag guessing game begin #martinplace @@muftimilk

O' people of Australia, do not blame the hostage taker, Rather blame your own government for Massacring Muslims everywhere. #sydneysiege Mujahid Miski @Muhajir_1436_

#BREAKING Hezbollah is behind this terrorist attack, in #sydneysiege State of Islam @Dawla__accountt

All those dummies who were celebrating the hostage taking of a nonviolent man for his views are now crying that Sydney hostages are innocent Shami Witness, Jr. @ ShamiWitness_Jr

Not that I condone attacks on chocolate shops. But when you launch a war against an enemy who hasn't attacked u expect retaliation. Shami Al Arabi (س) @AlArabii_

The police, however, are yet to determine the significance of the flag.

"We are still trying to work what the flag stands for," the New South Wales police force said in a media conference.