Chieng Bom Commune People's Committee announced on Thursday morning that a large fire has spread in a local forest since Wednesday evening.
"The fire started at 10 pm on March 6 and had yet to be brought under control as of this morning," the committee said, adding that the forest has a total area of 16,200 hectares.
Another fire also broke at 11 am on March 6 at a forest in Tong Co Commune. About 300 people joined in rescue work and managed to bring it under control by 3 pm.
A day earlier, a fire destroyed some 20 hectares of rubber plantation in neighbouring Muong La District.
Son La and other northwestern provinces have seen hot dry weather, which poses a high risk of forest fires.
Local authorities have been asked to closely monitor the risk of fires in forests in the area.