Police have cordoned off the area, along with other major landmarks in Sydney. Picture: Don Arnold/Getty Images) Source: Getty Images

UK Prime Minister David Cameron and US President Barack Obama have both been briefed on the hostage situation unfolding in Sydney’s Martin Place, as the dramatic siege makes headlines around the world.

White House officials confirmed President Obama’s top counterterrorism adviser Lisa Monaco briefed him on the event, after the family attended an annual Christmas in Washington concert. The White House is yet to release a statement on the issue.

MORE: LIVE COVERAGE OF HOSTAGE SITUATION

Meanwhile, leaders from New Zealand, Canada and India have expressed solidarity with Australia as the dramatic siege is picked up by media outlets from the BBC to Al Jazeera and The New York Times.

Motivation for the attack, which has been unfolding since 10am today, is still unknown. Earlier, Australian Prime Minister Tony Abbott said his heart goes out to the hostages and family involved.

“You have to appreciate that even in a society such as ours there are people who wish to do us harm,” he said.

“This is a very disturbing incident and I understand the concerns and anxiety of Australian people ... There is almost nothing more distressing and terrifying.”

Dramatic scenes filmed by Channel 7 from across the street. Source: Supplied

New South Wales Police Commissioner Andrew Scipione said the scenario is “consistent with a terrorism theme”.

Police have not yet established direct contact with the attacker, but Mr Scipione said he is confident the situation will be resolved with a positive outcome.

“There are a number of incidents people believe may be connected to city event and that’s not the case ... We are very contained. We certainly have tight restrictions and tight controls around that location. We will work as long as we need to.”

Gunmen believed to be members of ISIS have taken hostages at Lindt cafe in Martin Place. Source: News Corp Australia

Police outside the cafe where the siege is taking place. Source: Supplied

Australian National University’s terrorism expert Clive Williams said the attack appeared to be politically motivated, with the attacker using the strategy made popular by the Islamic State.

“It looks to be publicity orientated. Hopefully police will be able to negotiate the situation to a peaceful conclusion after a few hours,” he said.

“It seems to be consistent with what the Islamic State has been urging its followers to do, which is to conduct low tech, high impact acts in Western countries. The aim is to put pressure on governments to draw down their forces in the Middle East or withdraw them.”

Earlier, dramatic vision filmed from Channel Seven headquarters directly opposite the cafe showed several hostages pressed against the window with a black flag inside. It’s thought there are up to 50 people trapped inside.

An armed policeman is seen on Philip St Picture: by Mark Metcalfe/Getty Images. Source: Getty Images

Large parts of the city have been blocked off, with the Opera House also evacuated. Picture: Don Arnold/Getty Images. Source: Getty Images

Police intercept a boat that was approaching the Sydney Opera House. Picture Craig Greenhill Source: News Corp Australia

Mr Williams said how events unfold depends on whether police can negotiate with the attacker.

“Presumably they can do that through the phone, then it’s a matter of negotiation.”

Mr Williams said if the attack was religiously motivated police may bring in a senior Muslim leader and it could go on “for quite some time”.

The attack has prompted a large media response however Mr Williams said this may be unwarranted.

“It appears to be low tech and may not warrant kind of reaction measures being adopted around Sydney. It sounds like, from media reports, there’s an American-style reaction. The Americans will close down a whole city if they’ve got an incident, whereas we usually adopt a more measured approach.”

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People gather in Martin Place to try and see the siege at Lindt cafe in Martin Place. Picture Cameron Richardson. Source: News Corp Australia

The Opera House has also been evacuated. Picture: Braden Fastier Source: News Corp Australia

Aerial view of Sydney CBD where a siege is in progress. Picture Craig Greenhill Source: News Corp Australia