Sydney cafe siege live
Two hostages (L) run for cover behind a policeman (R) during a hostage siege in the central business district of SydneyWILLIAM WEST/AFP/Getty Images

A gunman who has taken an unknown number of hostages at a Sydney café has been identified by police and is known to them, it has been reported.

New South Wales state police Commissioner Andrew Scipione refused to elaborate on ABC Australia report that the hostage-taker is known to authorities.

"We won't go into the details about who is in there, how many are in there. We are not in a position to do that at the moment," Scipione told a press conference.

Scipione said Police "number 1 priority "is to get to safety the hostages at the Lindt Chocolat Cafe at Martin Place, central Sydney.

"Our only goal tonight is to get these people that are currently caught in that building out of this safely," he said.

Scipione said the "best negotiators in the world" were discussing with the gunman and sent a direct message to the hostages.

"Rest assured we are doing all we can to set you free," he said.

We have large number of police they're in contact and working to resolve this as quickly as we possibly can," he said.

Australian Prime Minister Tony Abbott said the gunman who stormed the café on Monday morning (15 December) claimed to have a political motivation.

However refused to comment on reports suggesting the attacker might have been moved by radical Islamic views.

"We are not in the business of speculating we are in the business of investigating," Scipione 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.

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 and police would not comment on the numbers.

The Channel 7 journalist Chris Reason, who has a view of the premises, tweeted he saw about 15 people inside the café.

Australian authorities have asked the media not to disclose the gunman's requests as negotiations are underway.

They also said they are monitoring social media, after a list of demands appeared on the accounts of some hostages.

The hostage taker has reportedly made several hostages call local media outlets, including ABC and Radio station 2GB.