A house was destroyed in an explosion on Church Road, Seven Sisters in the Neath valley, damaging neighbouring properties and vehicles.
One adult and two children have been seriously injured. Both children were flown by air ambulance to Southmead Hospital, Bristol, and the adult was taken to Morriston Hospital, Swansea.
Fourteen properties were also evacuated following the incident and Neath Port Talbot Council have opened a rest centre with full support for these residents.
A South Wales Police spokeswoman said the cause of the explosion was being investigated and that “the road will remain closed overnight and diversions are being put in place.
Mid and West Wales Fire and Rescue Service confirmed that crews were called to the scene at 14:05 BST.
Utility company Wales and West Utilities also said that it had sent emergency engineers to assist.
“We are working with the emergency services to make the area safe and to investigate the cause of the explosion,” said gas emergency service manager Sarah Burgess.
The road will remain closed overnight and diversions are being put in place.
Around 14 properties have been evacuated and Neath Port Talbot Council have opened a rest centre with full support for these residents.
The cause of the explosion is under investigation.