The Calgary police chief says he accepts responsibility after a 12-year-old boy was bitten by a police dog causing puncture wounds Wednesday evening.

The dog broke free from its backyard in Aspen Hills and came across three children playing on the street around 6 p.m.

A 12-year-old boy ran away from the dog and the dog chased him to his nearby home.

The dog bit the boy on the leg causing puncture wounds, while the boy's father tried to get the animal off his son just as the dog's handler arrived.

The handler gave first aid but the boy was taken to hospital, treated and released.

"Obviously this is terrible," said Chief Roger Chaffin at a news conference Thursday afternoon.

"When you imagine a dog coming into your house and attacking your son ... in front of your family, there is a lot to absorb."

Chaffin says the dog, a Belgian Shepherd named Marco, graduated from the training program about a year ago.

Marco has been held at a secure kennel after being removed from service. (Calgary Police Foundation 2016 Canine Unit Calendar)

The chief says the force will conduct a thorough investigation to learn what happened and if policy changes are needed.

While the solicitor general, director of law enforcement in the province and the Alberta Serious Incident Response Team have been notified, the investigation is currently being conducted by Calgary police.

"I accept the responsibility on behalf of the organization for what occurred," Chaffin said.

"I promise that I will work to get the answers to the questions that [the family] and other members of the public will have."

The dog is currently in a secure kenneling facility while the investigation — expected to take a few weeks — continues.