Police and fire service personnel are currently attending a fire at Altnagelvin Hospital in Derry.
The entrances to the hospital have been closed to all non-emergency personnel
Police have confirmed that the evacuations of several wards has begun.
There are no reports of any injuries at this time.
It is thought the fire may have started on the hospital's roof leading to the collapse of water tanks. Floors 9 and 10 have been effected.
The Western Health Trust says that the evacuation is being facilitated by Translink and their bus service.
The trust, in a statement, added that it was "reassuring the public that there are no injuries and the incident is under control".