Two children and a woman were seriously injured after an explosion at a house in Seven Sisters, Neath Port Talbot.
Emergency services including police, ambulance, air ambulance and fire service were called to Church Road shortly after 2pm Wednesday afternoon.
The two boys, aged 2 and 5, were taken by air ambulance to Southmead Hospital in Bristol, while a 31-year-old woman was taken by ambulance to Morriston Hospital.
They all remain in hospital and are said to be in a critical but stable condition.
South Wales Police described it as a "serious incident" and said a number of properties and vehicles in the street have been damaged.
Church Road remains closed and around fourteen nearby properties were evacuated. Police are asking people to avoid the area.
A rest centre was set up by Neath Port Talbot Council offering support to anyone affected.
Kirsten Alison Williams, who lives locally in the area, told ITV Wales she heard a "massive bang."
"We were sitting in the garden on the patio area, and then the conservatory started shaking and there was a big, massive bang", she said.
"I've never ever experienced anything like that, you could feel it in your stomach."
Watch: Pictures show extent of the damage following the explosion in Seven Sisters
The cause of the explosion is under investigation, but a spokesperson for the gas emergency service at Wales & West Utilities said a team of emergency engineers had been sent to assess the scene.
Sarah Burgess, Wales & West Utilities Gas Emergency Service Manager for Neath said: “We were called yesterday to reports of an explosion at a property in the Church Road area of Seven Sisters (24 June) and immediately sent a team of emergency engineers to the scene.
“We worked with the emergency services to make the site safe. We do not operate the gas network on Church Road and have handed it over to the relevant independent gas network to continue the investigation.”
Councillor Rob Jones, leader of Neath Port Talbot Council, said: "I was shocked and upset to hear the distressing news from Seven Sisters this afternoon.
"My thoughts and those of my council colleagues are with those who were injured in the explosion, and we hope and pray that they make a full and speedy recovery.
“I know that Seven Sisters is a close-knit community, and that many will be feeling a mixture of shock and sadness at this time.
“I would like to thank the emergency services for their swift response and regular updates on the situation. The council stands ready to offer whatever support is required.”