Neighbours described hearing "horrible" screams as twin four-year-old boys were trapped amid exploding gas cylinders in a burning garage in suburban Melbourne last night.

Paramedics say the twins were playing in the garage in Fordholm Road, Hampton Park, shortly after 7:00pm when there was an explosion and their clothes caught fire.

One boy suffered burns to 80 per cent of his body while his brother has burns to 50 per cent of his body. Both were in a critical condition in Melbourne's Royal Children's Hospital this morning.

Their father was also taken to hospital with minor burns which he suffered while trying to extinguish the blaze.

Neighbour Jessica Davies says it was a dreadful scene when they discovered the fire and heard the boys screaming.

"It was just black and the screams were just horrible, there was nothing to describe how bad it was, it was absolutely horrible," she said.

Her boyfriend, Travis Graeske, tried to rescue the boys but says the fire was too intense.

"The closer we got, the hotter it got, and there were just explosions going off, gas cylinders exploding and you just couldn't get near there," he said.

"That was the worst part, knowing you couldn't do anything for those little kids."

Mr Graeske says the boys suffered horrific burns.

"Head to toe, just covered. Skin peeling off," he said.

"One of the next door neighbours just grabbed him in her arms, and rushed him to a tap and we just chucked as much water on him as we could."

Ms Davis says her boyfriend was not hurt but is very shaken up.

Police are still investigating the cause but do not believe it was suspicious.