Patients on five floors at Altnagelvin Hospital in Londonderry were evacuated because of a fire on the roof last night.
Almost 50 firefighters in 11 appliances were called to tackle the blaze which broke out at about 6.20pm.
Northern Ireland Fire and Rescue Service group commander Edward Carroll said patients had not been in any danger.
Mr Carroll described the fire as small but said its location on the upper levels of the 11-storey hospital building had warranted a large scale operation.
"The actual fire is not a major fire," he said. "The situation is quite fluid. Our crews are still at the scene. But we have a confirmed fire on the roof of the hospital. That is ongoing. We have had a controlled evacuation of floors five to 10.
"We have a total of 11 appliances including specialist appliances and 49 officers at the scene.
"It is classed as a high rise fire and that presents a number of challenges."
Mr Carroll said the cause of the blaze was under investigation.
Among the 11 appliances called out from Derry's two fire stations and Strabane was a specialist unit carrying aerial equipment.
A spokeswoman for the Western Health Trust also moved to reassure members of the public that the situation was under control.
In a statement the trust said: "The Northern Ireland Fire and Rescue Service and Northern Ireland Ambulance Service are on site assisting with the evacuation of a number of wards.
"Currently evacuation is being facilitated by Translink and their bus service.
"Patients in the affected wards are being transported to appropriate hospital or other care settings."
A PSNI spokesman confirmed that police were also at the scene.
The entrances to the hospital at Glenshane Road have been closed to all non emergency personnel.
SDLP MLA for Foyle, Mark H Durkan, who was at the scene, praised the emergency services for their response.
He said: "This is a very serious incident, which must be causing untold distress to patients and also to staff. I wish to commend the emergency services for their fast response and tireless work to alleviate the situation and bring the blaze under control."
Northern Ireland Health Minister Edwin Poots also praised the emergency services.
The DUP MLA added: "I have been receiving regular updates on the situation in Altnagelvin and, while I understand that it has been difficult for patients who have been evacuated, I am relieved to hear that no one has been injured.
"I would like to thank NIFRS, the Ambulance Service, PSNI and hospital staff for their swift reaction to dealing with this fire."