"Obviously there are assessments and discussions ongoing with the fire service and building control about the structure of it and it's structural safety".
It was regarded as his masterpiece, and officials had spared no expense in the restoration that began after the 2014 fire.
Scottish Fire and Rescue Service (SFRS), who were still working to extinguish the flames on Saturday morning, said no-one was killed or injured in the incident.
Scottish First Minister Nicola Sturgeon said she was heartbroken by the fire, tweeting: "This is clearly an extremely serious situation".
Local resident John McInnes, 22, said: "I can't believe that this has happened again to such an iconic building".
The blaze engulfed the Glasgow School of Arts and spread to neighbouring buildings.
The building was built in the late 1890's by Charles Rennie Mackintosh, then a junior draughtsman, and is widely considered to be his masterpiece.
The Glasgow School of Art issued a statement about the fire today, noting that the building-affectionately known as "the Mack"-hasn't been in use for four years due to restoration efforts following the previous blaze".
Police Scotland advised the public to avoid the area, and said areas around Dalhousie St, Scott St and Sauchiehall St were all affected by road closures.
A restoration project, set to cost between £20 million and £35 million ($26.5 million and $46.5 million; 23 and 40 million euros), had been returning the world-renowned institution to its former glory following a fire in 2014.
"Questions need to be asked, they spent so much on renovation, but this seems so much worse than the last one".
Writing on Twitter, Paul Sweeney, a Glasgow MP, said: "It looks like the entire interior space is now fully alight".
Charles Rennie Mackintosh, one of Scotland's most influential figures, inspired designers, architects and artists around Europe with his distinctive style.
The UK government "stands ready to help, financially or otherwise", he added.