Fire crews were called to tackle a blaze at the old Crumlin Road Courthouse this morning.
The incident, which is being investigated as deliberate, started shortly before 3am on Monday morning, and was ongoing for several hours.
Firefighters remain at the scene and say they will be there for some time.
Brian Stanfield, NIFRS Area Commander told Belfast Live: "We received the call at 3am this morning and four pumps and one aerial appliance were involved. The fire started in the old courtroom of the building.
"Our crews remain at the scene and will be there for quite some time. There was no spread to local residential properties.
"An investigation is now underway and we are treating this as deliberate at this stage."
Assistant Chief Fire & Rescue Officer, Paddy Gallagher, said: “This was a significant fire that came at the end of an extremely busy weekend for Northern Ireland Fire & Rescue Service and I want to thank all of our Firefighters who have stepped up to the mark over the last couple of days, as well as all of our partner agencies who have worked closely with us in responding to a number of challenging incidents.”
Police in North Belfast say they are now investigating the incident.
A spokesperson said: "A report was received at around 2.55am on Monday morning of a fire at the building. NIFRS attended and extinguished the fire which is being treated as deliberate at this time.
"Officers would ask anyone with information which could assist them with their enquiries to contact police at Tennent Street on 101 quoting reference number 213 01/06/20. Alternatively, information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime.
"You can also make a report using our online non-emergency reporting form via http://www.psni.police.uk/makeareport/."
