SYDNEY, Dec. 15  -- A gunman, who held an unknown number of people hostage in a downtown Sydney cafe on Monday, has been identified after five hostages managed to flee the hours-long siege.

The Australian Broadcasting Commission (ABC) reported that the man was known to police, but it has been told not to reveal his name.

Three men and two women, who sprinted from the Lindt Chocolate Cafe, told the media that the man had warned them that two bombs were set to explode in the cafe, and other bombs were planted at two locations in the city.

The armed man also demanded the delivery of an Islamic State ( IS) flag and a conversation with Australian Prime Minister Tony Abbott.

The escapees told Channel 10 TV that the man forced his captives to call him "brother" and would release a hostage if the flag was delivered.

The siege began at 9:40 a.m. local time (2240 GMT), and has involved hundreds of armed police who surrounded the cafe located at the Martin Place in Sydney's central business district.

The hostages have been forced to hold a black and white Islamic flag on the cafe window that says: "There is no god but Allah, Muhammad is His messenger."

Shop staff in aprons could be seen with their hands on the windows.

Nearby streets were cordoned off and all trains were shutdown near the area.

The police also evacuated the Sydney Opera House and shut down traffic on the Sydney Harbor Bridge.

The government convened a meeting of the National Security Committee this morning. It has been warning for months that a terror attack could happen in Australia.

"We don't know whether this is politically motivated, although obviously there are some indications that it could be," Prime Minister Abbott said in the capital of Canberra.

After the anti-terror raids in September, he said all that was required for a domestic IS-ordered terror attack was a "knife, iPhone and a victim."

New South Wales police commissioner Andrew Scipione told reporters that the police were doing all they could to resolve the situation peacefully.

"The officers there are well trained. They know what they are doing," he said, adding that there were tight controls around the location, and the police had not made contact with the gunman.

Scipione said the police still did not know the his motivation and were working to determine where he was from.