SA police vigilant in wake of Sydney siege

South Australian police patrols have been asked to be particularly vigilant as a hostage drama continues at a central Sydney cafe.

But SAPOL has urged the community to go about their business as usual.

"If SAPOL becomes aware of matters impacting community safety and security, the public will be alerted," police said in a statement.

"SAPOL continue to work with other police jurisdictions and Commonwealth security agencies to ensure all necessary steps are being taken to keep our community safe."

Up to 50 people are believed to have been taken hostage at the Lindt Chocolat Cafe in Sydney's CBD, where an Islamic flag has been pressed up against a window.