Miri Fire and Rescue Department chief Supt Law Poh Kiong said the fires were inside an oil palm plantation.
"It is huge. It is burning beyond control.
"We, Bomba Miri, are sending in our aerial water-bombers now," he said in his latest report to Sarawak Bomba.
The fires, 30km north of Miri city in the Kuala Baram district, started on Monday (April 15) and have blackened the surrounding skies.
They can be seen 10km away in the South China Sea.
An offshore worker at an oil-rig in the South China Sea took pictures from the platform that showed the smoke billowing kilometres away in the distance.
Supt Law said the fires were located in forests that were difficult to reach by land.
"That fire is very big and there is no road access. That makes it very hard to reach by land," he said.
As at 2pm on Tuesday (April 16), the fires are getting from bad to worse.