Underbank Reservoir. Picture: Google.

The boy was attacked while walking along the Underbank Reservoir, near the Stocksbridge Bypass, at around 12.15pm on Sunday.

Police said a man and woman were walking four dogs along the reservoir when the incident happened.

Underbank Reservoir. Picture: Google.

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One of their dogs, which was off its lead, ran up to the boy and attacked him, South Yorkshire Police said.

The boy sustained injuries that resulted in him needing surgery.

The owners gained control of their dog, stopped to assist the victim and tried to leave their contact details, however due to the nature of the incident, the victim’s grandmother did not manage to record them.

Officers said they would like to reassure the public that this was an isolated incident and asked anyone that can help provide information to get in touch.

The dog was described as being a large build mastiff cross breed.

The owners were described as being aged in their 50s.