Police have appealed for urgent public help to find a baby boy who has been abducted at knifepoint from a home in Sydney's west.


The armed man forced his way into a home on Kenthurst Place in Chester Hill about 8.30pm on Thursday and abducted the eight-month-old boy and a 16-year-old girl.


The man, who had tattoos across his arms and chest, put the pair into his black Toyota Camry and drove away.


Police said about 10pm the car stopped on the Hume Highway at Douglas Park where the teenager was assaulted.


The man then left the teenager by the side of the road and drove off with the eight-month-old boy in the car. Police have not revealed whether the man knew the teenager or the baby.


Police are now searching for the man and his vehicle, which is a 2007 black Toyota Camry with NSW registration AN-22-XX.


The man is between 165 and 175 centimetres tall, with a thin build, brown hair, and was wearing a blue jumper and green tracksuit pants. He has tattoos on his arms and chest.


Police have asked anyone who sees the man or his vehicle to call triple-0 immediately.