U.S. President Barack Obama has been briefed on the hostage situation in Sydney's central financial district, where a gunman seized an undisclosed number of hostages at 9:44 a.m. local time (5:44 p.m. EST, Sunday), at the Lindt Chocolat Café on Monday.

According to White House officials, Obama has been briefed by his top counterterrorism adviser, Lisa Monaco, who is also the assistant to the president on homeland security issues. The U.S. also evacuated its Sydney consulate, located near the café and issued an emergency warning to American citizens in Sydney, urging them to “maintain a high level of vigilance and take appropriate steps to enhance personal security,” ABC News reported.

The Lindt Chocolat Café has also issued a statement about the hostage situation on its Facebook page:

We would like to thank everyone for their thoughts and kind support over the current situation at the Lindt Chocolate Café at Martin Place. We are deeply concerned over this serious incident and our thoughts and prayers are with the staff and customers involved and all their friends and families. The matter is being dealt with by the authorities and we are waiting for any updates from them.

It's yet unclear who is responsible for the incident, but according to reports citing authorities in New South Wales, local law enforcement is addressing the situation as if it were a terrorist threat.

Meanwhile, leaders from New Zealand, Canada and India have expressed their concerns about the dramatic siege.