Families had to be moved from their homes in the early hours after a suspected arson attack in Coleraine.
Up to 30 firefighters were called to the block in Long Commons after a fire broke out in a second floor flat just after 2am.
Firefighters managed to stop the fire spreading to the other flats.
An investigation into the cause of the fire is under way, but the fire service believe it was started deliberately.
Fire service Group Commander Tom Morrow said there was substantial damage to the flat where the fire started.