Man charged with raping missing teenager he detained for four weeks

A man has been charged with raping a 15-year-old girl whom he had allegedly held against her will for a month at a house in Sydney's west.

The teenage victim had been reported missing from a residential out-of-home care facility this month.

About 1.30am on Thursday, police saw her being chased down Balmoral Street, Blacktown, allegedly being chased by the man. 

Officers allegedly saw the man grab the girl and try to drag her away before they intervened and took the man to Blacktown police station.

The girl told officers the 29-year-old had been holding her against her will and sexually assaulting her for four weeks at a house in Blacktown.

"Inquiries also revealed the girl had been reported missing from a residential out-of-home care facility earlier this month," police said in a statement.

"She was taken to The Children's Hospital at Westmead for medical assessment and has since been released."

The Department of Family and Community Services have been told of the incident, police said.

Officers raided the man's house and seized a number of items.

He was charged with aggravated sexual assault, common assault and was refused bail to appear in Blacktown Local Court on Friday.

"Police also notified the Department of Immigration and Border Protection, regarding the man's arrest," police said.

Inquiries into the incident are continuing.