Hundreds of firefighters tackled forest fires causing toxic smog in the 'world's most polluted city' in Thailand.

More than 200 personnel were sent to the Pha Daeng National Park to battle wildfires raging in Chiang Mai on March 30.

Kritsayam Kongsatri, the director of Protected Area Regional Office 16, said the blaze, which started in Ban Kae Noi village and spread to nature reserves in the Chiang Dao district, was the 'most severe' Chiang Mai has experienced.

He added that the smoke was adding to the already alarming air pollution, with Chiang Mai ranking worst city for air quality in the world, according to Swiss air monitoring company IQAir this week.

Kritsayam hoped the fire will be brought under control by today.

He said: 'Reconnaissance teams were dispatched to continuously monitor and prevent any potential re-ignition, particularly in the risky areas including the Chiang Dao Wildlife Sanctuary.

'The Chiang Dao Wildlife Sanctuary is a high and steep cliff, making it one of the most dangerous places to extinguish fires in Chiang Mai. Close surveillance is required.'

The Geo-Informatics and Space Technology Development Agency yesterday detected some 343 hotspots in northern Thailand.

Around 1,670 firefighters have been deployed to keep the fires at bay.

Northern Thailand has been grappling with numerous forest fires in recent weeks. The flare-ups are believed to be caused by agricultural burning, as farmers cheaply clear their land by setting ablaze waste crops.

It has led to soaring levels of harmful PM2.5 particles in the atmosphere, with air pollution in Chiang Mai and Chiang Rai the worst in the world this week.