Hostages being held in a siege at a Sydney café have posted their captor's demands to authorities on social media, it has been reported.

A tense standoff with Australian police erupted on Monday morning (15 December), as a gunman stormed the premises of the Lindt Chocolat Cafe at Martin Place, central Sydney, taking numerous hostages.

Hours into the crisis, two hostages made a dramatic phone call to Australian outlet Ten Eyewitness News, in which they detailed information about the hostage-taker and his demands.

Australian authorities have asked the media not to disclose the gunman's demands as negotiations are underway. They also said they are monitoring social media, after a list of demands appeared on the accounts of some hostages.

Ten Eyewitness News reported Hugh Riminton, who spoke with two of the captives, tweeted: " 2 bombs in #lindtcafe" and "two bombs have been placed in the CBD (central banking district)."

He added that the two women in the call were "hysterical" and believed that their lives were in "immediate danger".

Riminton then deleted the tweets as they included the demands of the hostage-taker.

Five people have managed to escape since the crisis began more than six hours ago. It was not clear how many people remained inside the café.

The Channel 7 journalist Chris Reason, who has a view of the premises, tweeted:

Australian Prime Minister Tony Abbott described the incident as "very disturbing", adding the hostage-taker claimed to have a political motivation.

"It is profoundly shocking that innocent people should be held hostage by an armed person claiming political motivation," Abbott said.

Earlier, the gunman, described as a bearded middle-aged man wearing a white shirt and a bandana, reportedly forced two hostages to hold up an Islamic flag.