TAI Calon Community Housing is reporting a gas mains pipe burst in Nantyglo last night, which is now thought to be affecting around 4,500 homes in the area.
Water is said to have gotten in the gas pipework.
Tenants are urged not to use any gas appliances and to switch of their boiler.
A Tai Calon spokeswoman said: "This is important to prevent damage/further damage to appliances. We are diverting staff to help deal with the situation.
"We know that homes in Waenheulog, Roundhouse Close, Brynawelon, Farm Road, Waen Fawr and Waen Ebbw bungalows are definitely affected."
The latest information is that up to 500 homes have been affected in Nantyglo.
Tai Calon staff are in the area offering help and support to tenants.
The community housing organisation has advised Waunheulog residents to switch off the gas supply to their home.
A Wales & West Utilities spokesman has confirmed that 500 properties in the area are affected.
He said: “The gas supply has been interrupted to approximately 500 properties in the Nantyglo area, due to water getting into our gas pipe from a burst water main nearby.
“We are working alongside Dwr Cymru Welsh Water in resolving this incident, and our engineers are currently working to remove the water from the gas pipe – which can be complex. We are as yet unable to let people know how long this work will take, but we will keep the community updated throughout the afternoon.
“We also have engineers going door to door to make sure that gas supplies to properties affected are turned off. We will then be able to start work on restoring supplies to the affected properties. Once this is completed, our engineers will be visiting each property to safely turn their gas supplies back on.
“We are working as quickly and as safely as we can, and will be monitoring the situation throughout the day. Depending how long this work takes, we will be offering vulnerable customers alternative cooking and heating facilities.
“If people need to contact Wales & West Utilities, our customer services line is 0800 912 29 99.”