A number of homes have been evacuated in the Parkmore Close area of the Co Antrim town, as part of the security operation which began on Sunday afternoon.
Parkmore Close, Parkmore Drive, Parkmore Heights, Upper Princes Street and the Cullybackey Road have all been closed and diversions are in place.
Translink have said the railway line between Antrim and Ballymoney has been closed due to the security alert and a bus substitution is in operation.
Police and army bomb experts remain at the scene.
A PSNI spokesman said: "It is anticipated the alert will go on into the early hours of Monday morning."