A man has had a finger severed after two intruders attacked him when he went to investigate noises in the back yard of his western Sydney home.

Police say the 44-year-old was struck with an unknown weapon in the early hours of Saturday at his Shalvey home before his two attackers fled on bicycles.

Police and paramedics were called to the house at about 1.50am and after being treated at the scene the man was taken to Westmead Hospital with a severed finger and partially severed finger.

A crime scene has been established and police are urging anyone with information on the incident to contact them.

© AAP 2014