Fifty firefighters, six fire appliances and specialist firefighting equipment were deployed to tackle a "very significant" blaze at Belfast Harbour estate on Tuesday.
The fire broke out at a scrap metal recycling business in the Queen’s Road area at about 1pm, and was expected to continue burning for some time.
Large plumes of smoke were seen billowing across parts of the city on Tuesday afternoon.
The Police Service of Northern Ireland (PSNI) said residents living nearby should keep their windows closed, and motorists have been advised to avoid the area.
Both the Northern Ireland Fire and Rescue Service (NIFRS) and the PSNI remained at the scene on Tuesday evening.
NIFRS Group Commander David Harbinson told the BBC firefighters were working in "very difficult conditions" to bring the fire under control but it would be a "protracted incident" which was expected to last for a "number of hours".
He said there were sufficient resources on site to deal with the incident, which included the use of a fire tug, but the nature of the fire made it “inherently very difficult to deal with because of the pile of scrap metal, they do maintain a lot of heat.
“That then allows the fire to spread, so whenever we start to attack the fire it can be very challenging for us.
“It does require vast amounts of water, and again securing those water supplies can be very, very difficult.”
He said the fire service expected to be at the scene “for some time” and appealed to the public to “stay away from the scene to allow us to get on with our operations”.
