CANBERRA, Dec. 15  -- Australian Prime Minister Tony Abbott said on Monday that the hostage drama occurring in Sydney could be politically motivated, saying he could think of "almost nothing more distressing or terrifying" than being caught up in the siege.

Speaking with the media outside the Parliament House on Monday afternoon, Abbott said it was a "disturbing incident" and that the New South Wales Police, the Australian Federal Police and the government were currently investigating the motivations behind the siege.

"We have to appreciate that even in a society such as ours, there are people who would wish to do us harm," Abbott said.

"That's why we have police and security organizations of the utmost professionalism that are ready and able to respond to a whole range of situations and contingencies including this situation that we are now seeing in Sydney.

"The whole point of politically motivated violence is to scare people out of being themselves. Australia is a peaceful, open and generous society. Nothing should ever change that and that's why I would urge all Australians today to go about their business as usual."

As of 1.00 p.m. local time, a person carrying a firearm remains inside a caf in Sydney's Martin Place with several hostages, who have been seen holding up a black flag bearing Arabic writing.

The siege began at 9.45 a.m. local time.

While heavily armed NSW Police move into position in surrounding areas, parts of Sydney have been forced into lockdown.

"I can understand the concerns and anxieties of the Australian people at a time like this, but our thoughts and prayers must above all go out to the individuals who are caught up in this," Abbott said.

"I can think of almost nothing more distressing or more terrifying than to be caught up in such a situation and our hearts go out to these people."

Meanwhile, the White House has confirmed that U.S. President Barack Obama has been briefed on the ongoing situation in Sydney, while the U.S. Consulate has released a statement regarding American citizens in Australia.

"U.S. citizens are strongly encouraged to review your personal security plan, remain aware of your surroundings including local events and monitor local news stations for updates," said the statement on Monday.