House fire raged for up to 30 minutes before three children found inside

A house in Melbourne's north-west was believed to be on fire for 20 to 30 minutes before firefighters arrived and found three young children unconscious and critically injured inside.

Five-year-old Izabelle, three-year-old Kalais and their 21-month-old sister were rescued from the burning home in Sydenham about 9.30pm.

Firefighters arrived at the home about six minutes after neighbours first called triple zero after hearing a "loud bang" coming from the home.

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Detective Acting Inspector Adam Henry said firefighters were not aware anyone was home when they arrived, but found the siblings inside the single-storey home and rushed them to safety.

He said all three children were all unconscious, having suffered from smoke inhalation, and were still in a critical condition in hospital this afternoon.

"This is an incredibly tragic incident," he said.

"Firefighters in particular did an incredible job to locate the children and get them out of the home quickly."

Kalais' father, Jayde Petalas who doesn't live at the house, said he was heartbroken after receiving the call from police about the fire.

"They were the best little kids. They would have been petrified," he said.

"I hope they didn't feel anything to be honest. Obviously they would have, but I just tell myself they didn't."

Henry said it was understood the children and their mother had moved into the house just last week, and police were probing whether she was home at the time of the fire.

He said police had only been able to speak to her briefly due to her distress.

The cause of the fire is still unknown, with Henry saying that while neighbours had believed they heard fireworks, no evidence of any had been found.

"It is too early to say whether the fire is suspicious or not," he said.

"We'll work tirelessly to confirm the cause."

He said police had established the fire started at the back of the property.

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Henry said the children's mother was known to police, and they had spoken to one of the children's fathers.

A total of 30 firefighters were called to the fire and brought it under control about 11pm.

They remained at the scene for several hours afterwards.

Police could be seen today removing multiple items from the property, including children's toys.

A dog was also recovered from the home about 9.30am.

Neighbours told 9News that they did not realise anybody lived at the home, which was recently sold in June.

"Nobody thought there was anyone inside," one said.

"There were never really any people there or anything really going, it only recently sold.

"We have never heard anything like it. It was like a bomb dropped," she said.

"The flames were bright orange and it was intense. There was an orange glow.

"My husband went outside and said it looked like it was getting worse and there were kids inside and that was really hard."

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Another neighbour described the fire as "heartbreaking".

"I had no idea who was in there, the only thing I could recall was hearing those kids playing outside (over the weekend)," she said.

"To hear they are fighting for their lives is heartbreaking.

"It's just awful, I work with kids and to hear that is heartbreaking. I can't imagine what their parents are going through."

All three children are still fighting for life at the Royal Children's Hospital.

Anyone with information or who may have witnessed the incident is urged to call Crime Stoppers on 1800 333 000.