Armed man on the run after targeting women walking alone in Sydney

A man armed with a knife and hammer is on the run in Sydney's north-west after targeting and stealing from three women walking alone.

Police received a number of reports of robberies and assaults in the Northmead and North Rocks area early this morning, which sparked a manhunt and forced a local school into lockdown.

An extensive search of the area - which involved a helicopter, specialist officers and the dog unit - has failed to find the offender responsible.

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At a press conference this afternoon, Superintendent Barry Vincent from NSW Police said the spree of attacks began when a white Mitsubishi Outlander SUV was reported stolen from a business on Windsor Road in North Rocks about 5am.

Officers soon received another call that a 47-year-old woman walking alone on Mons Road in Northmead had been approached from behind by a man.

"After a short wrestle, her personal items including a bag were taken from her," Vincent alleged.

The offender then hopped in a SUV - believed to be the same one reported stolen from North Rocks - and fled the scene.

While officers were responding, they were called to Junia Avenue in Toongabbie where a 19-year-old woman was approached from behind by a man of similar description.

"It's alleged he produced a knife and a number of personal items were stolen from this woman as well," Vincent said.

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A short time later, a 27-year-old woman was approached from behind in a similar fashion on North Rocks Road in North Parramatta and threatened with a hammer for her items.

Vincent said men known to the woman tracked her stolen phone to the nearby North Rocks Road and confronted the offender, who smashed their car window with a hammer and assaulted them before again fleeing the scene.

"We were unable to locate the offender," he said.

"A number of items, including the motor vehicle used during the offence, have been located and seized for forensic examination."

Vincent said while none of the women were seriously injured, the nature of the alleged offending was of "deep concern" to officers.

"I would describe this as particularly cowardly, just targeting women walking alone in the early hours from behind," he said.

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CCTV obtained by 9News showed a masked man - who Vincent said matched the description of the offender - approach the front door of a home just before 4am today.

"Police are also investigating if the same man is responsible for break-ins committed on several homes in the same area two days earlier," police said.

The man is described as dressed in dark-coloured clothing, of a medium build and aged in his 20s.

If the man is sighted, police urge the public not to approach him and call triple zero immediately.

Anyone with information or footage is urged to contact Crime Stoppers on 1800 333 000.