(CNN) — A major police operation was under way Monday in Sydney where hostages are being held in a cafe.
CNN affiliate Seven Network said that at least 13 people are being held at the Lindt Chocolate Cafe.
The hostage taker has contacted three media outlets via the hostages and demanded two things: an ISIS flag and a phone call with Australian Prime Minister Tony Abbott, according to Sky News.
It published a photograph of people inside the building holding a black flag with Arabic writing on it. The flag reads: “There is no God but God and Mohammed is the prophet of God.”
Video showed people, presumably hostages, with their hands pressed up against the glass.
**See a statement from New South Wales police at the bottom of this page**
Australian Prime Minister Tony Abbott told reporters that authorities do not yet know the motivation of the person responsible for the ongoing situation.
“We don’t know whether this is politically motivated, although obviously there are some indications that it could be,” he said.
The Prime Minister added: “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.”
He said more information would be released as it becomes available.
Two U.S. officials briefed on the incident said that it was unclear exactly how many hostages are being held. There is at least one armed suspect involved, and so far, no known motive, they said. U.S. President Barack Obama was also briefed on the situation.
The cafe is at Martin Place in Sydney’s busy Central Business District.
Video showed police and emergency workers at the scene, where a number of nearby streets have been closed.
The Martin Place train station was shut, and some offices were evacuated as a precaution, according to police. They urged people to stay away.
Chris Kenny, an editor at The Australian newspaper, was at the cafe Monday morning. He left shortly before the siege started.
“As police quickly swarmed and cleared the area, I turned to see a man against the window, facing out with his hands raised.
“At first I was relieved thinking this was the gunman responding to police — but soon the awful realisation the customers were being forced against the windows,” Kenny wrote in the newspaper.
“Terror is in the heart of Sydney right now,” he said.
— NSW Police (@nswpolice) December 15, 2014