They have advised residents to stay inside their properties and keep all windows and doors closed.
Four fire crews are in attendance at the incident.
A number of people nearby are being moved from their homes. The Northern Ireland Fire and Rescue Service (NIFRS) said on Thursday night it appeared a contractor may have ruptured a gas main.
They received a call to attend the incident at Mull Road at about 21:40 BST.
"Thankfully it is an unignited leak, so it has not gone on fire," NIFRS Group Commander Dermot Rooney told BBC Radio Ulster.
"The police are managing the public safety by a cordon and by asking people to remain indoors and keep their windows and doors closed.
"We and Firmus Energy will be using gas monitoring equipment to monitor the concentrations of gas in the atmosphere, we also have fire appliances there in case the gas leak would ignite."
Police have said there is a "major gas leak" in the Stiles Way area of Antrim.