Four people are in hospital after an explosion and fire at a house in Llanbradach.
Gwent Police said one of them is in a "stable but critical" condition.
Another person has been released from hospital following the fire.
An eyewitness said the front door "blew off the house" in the the Dan-Y-Darren area. Police have confirmed the explosion was believed to have been caused by items within the property combusting.
South Wales Fire and Rescue said the injured were taken to hospitals across south Wales and Birmingham by road and air ambulance.
They said the incident has caused "significant damage" to the premises. A cordon is in place as a safety precaution.
Group manager Jason Evans said 25 firefighters were involved in tackling the blaze.
Wales and West Utilities said the explosion and fire was not related to the mains gas supply.