Dozens of police are involved in a siege at Raby, in Sydney's south-west.

Police say a man has barricaded himself inside a house on Sopwith Avenue with an elderly woman.

Police negotiators and the tactical operations unit have been at the scene since a call to triple-0 at about 9:00am (AEDT).

The ambulance service has assessed a woman outside who was not injured.

More to come.