Police are searching for a man who allegedly broke into an apartment in Sydney’s west and sexually assaulted the sleeping female resident.

Authorities were told that at about 1am on Tuesday, an unknown man broke into a unit in Blacktown and assaulted a woman in her 30s.

They were told that the man, aged in his 20s, entered the bedroom of the unit and sexually touched the female resident while she was asleep.

Police have established a crime scene and are searching for a man described as being of a thin build, in his 20s, standing at about 160cm tall with short hair.

As police continue to investigate, they have since learned of a string of incidents starting in December last year, where underwear was stolen off a clothesline, and a man was allegedly spotted peering through a window on the street.