Fire service Group Commander Alan O’Neill said firefighters have worked “tirelessly” all day to prevent the blaze at Primark spreading further.
But, he warned, the building itself is now at risk of collapse.
Speaking to the News Letter, Mr O’Neill said: “Whilst there is a considerable amount of damage to property, there was always the potential that it may have spread further.
“We do have some concerns that there is potential for collapse in the building, so we have been working with structural engineers throughout the day to assess and take every step we can to prevent further damage.”
He added: “We will continue to survey it and ensure that it is safe for people to go back into that area.”
Praising the efforts of firefighters, he continued: “The operation’s been scaled up to working with 14 fire appliances, three aerial ladder platforms, a high-volume pump and two command support units.
“And that has us working with nearly 100 firefighters on scene. They are certainly working tirelessly at the scene and there are a lot of physical operations to manage hose and get in through the building.
“But that’s what our crews are trained for. They are trained to deal with these things so they will be working away.”
The scale of the operation meant resources had to be moved around Northern Ireland to ensure the fire service could cope with emergencies elsewhere, he said.
“We have made, already, a considerable amount of contingency moves which is normal business for us when we do get a significant fire. We have moved resources around the country to make sure we can provide a response quickly to anywhere within Northern Ireland.”
The group commander praised those responsible for ensuring there were no casualties from the blaze.
“Whilst it is very damaging for the building itself and particularly for Primark, thankfully there hasn’t been any loss of life in such a significant fire,” he said.
“Praise must go to those who were on scene in the early operations for making sure that everybody got out quickly.”
Asked about the cause of the blaze, he said: “At this stage it would be too early to make comment but we will certainly work with Primark to investigate the site.”