Sydney woman assaulted in park

A 19-year-old woman has been sexually assaulted while on a run through a park in western Sydney.

Police say the teen was running through Duncan Park at Seven Hills at about 10am on Friday when she stopped at a bridge.

A man, armed with a knife, then allegedly grabbed her from behind and sexually assaulted her before fleeing.

The woman was taken to Westmead Hospital.

Police are now looking for a bearded man of Caucasian appearance.

He was last seen wearing light blue denim jeans, black work boots and a cap.

Anyone who has information is urged to contact Crime Stoppers on 1800 333 000 or Blacktown Police.