Four petrol stations held up in Sydney

An armed robbery at a service station in Sydney's inner west is the latest in a spate of robberies across the city.

A man armed with a knife threatened an attendant at Sydenham service station in the early hours of Friday and fled the scene with cash, police said.

The incident is believed to be linked to two other recent armed robberies in the area.

At about 2am on Thursday a man with a screwdriver tried to rob a Canterbury service station and 40 minutes later the same man, then armed with a knife, threatened staff at a nearby Ashfield service station and stole cash, police said.

On Thursday night in the city's west, a man armed with a rifle threatened staff and stole money from a Kings Langley service station, police said.