A man who stole a car at knifepoint from a driver who stopped to help him following a two-car crash in Sydney's west remains on the run.

At about 11.30pm yesterday, emergency services were called to Amy Street in Regents following reports of a two-vehicle crash.

Officers arrived to find a Volkswagen Ute and a Volkswagen Hatchback that had been involved in a crash.

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The 18-year-old female driver of the hatchback was uninjured. 

Police were informed that a 47-year-old man driving a van had stopped to help.

Police were told the driver of the ute then threatened the man with a knife before stealing his van.

"At this stage, we don't believe any parties are known to each other," Detective Chief Inspector Alison Fenwick said.

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"That's just lowlife," a local told 9News. 

"I really feel sorry for the poor bugger."

"That's disgusting," another resident told 9News.

"You shouldn't have to put up with that, trying to help someone." 

"It's as low as it goes."

The van was found shortly after, abandoned near Regents Park Train Station.

Both vehicles have since been seized by police for forensic examination.

"Anyone producing a knife and taking a vehicle is a serious crime. It will be investigated by detectives," Fenwick said.

"I'm confident we will identify who committed the crime, and action will be taken."

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