US president Barack Obama has been briefed about the Sydney siege situation, the White House says.

He discussed the situation with his top counter-terrorism advisor, Lisa Monaco.

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Ms Monaco is the assistant to the president for homeland security and counter-terrorism.

Dozens of heavily armed police have surrounded the siege site at a cafe in Martin Place.

The cafe is close to the Reserve Bank of Australia, commercial banks and the New South Wales state parliament.

Television footage showed several people inside the cafe standing with their hands pressed against the windows.