Images from the scene on Silk Mill Drive in Tinshill show that the whole upper storey of the property has been destroyed.
The explosion happened just before 3pm this afternoon. A man suffered injuries which are not thought to be life-threatening.
Stephen Marham, who lives three doors away from the property, said:
"I was upstairs talking to my wife. I just heard a massive bang and the house shook.
"My initial reaction was to run downstairs but as I got to the bottom I could see the flats. My partner just screamed 'the flat's gone'. I ran out of the house and saw a man struggling towards me.
"I could see he had been caught up in it. It was the man who lived in the flat. He was in a bit of a mess so I laid him on the floor in the recovery position.
"I took my jacket off and got a blanket. Another neighbour got one. He's been very, very lucky.
"There were two individuals in the house when it exploded.
"I went into auto-pilot you don't think about the consequences. I knew not to do anything heroic. I could smell gas after it had blown."