A helicopter made 20 trips to tackle a forest fire in northern Thailand this week.

The aircraft was deployed by the Digital Technology and Aircraft Centre to join ground firefighting efforts at the Op Luang National Park in Chiang Mai province on February 9.

As the terrain was steep and rocky, firemen had trouble reaching the area on foot.

Thailand's Department of National Parks, Wildlife and Plant Conservation said the fire has been brought under control.

The Department of Royal Rainmaking and Agricultural Aviation had launched cloud seeding missions in Chiang Mai a week earlier to mitigate haze and prevent wildfires.

Authorities have warned locals to stay indoors as air pollution in Bangkok and neighbouring areas reached unsafe levels.

Thailand's pollution control department blamed the rise in PM2.5 or hazardous inhalable particles on 'stagnant weather conditions' combined with agricultural burning in the north, northeast and in neighboring Laos and Cambodia.

Scientists found that smog is at its worst in the country when farmers burn waste crops to cheaply clear their land between late October and April. This is combined with high numbers of polluting cars on the road and large construction projects across the capital.

The seasonal lack of wind and rain, combined with a winter low-pressure system in which cold air is trapped closer to the ground by warm air above – prevent the pollution from being dispersed naturally.

The Thai government has been criticised for failing to tackle the country's air pollution crisis, with bizarre measures such as spraying water into the air and giant purifiers having no impact.

Data scientist engineer Worasom Kundhikanjana said: 'PM 2.5 pollution has a seasonal trend in Bangkok and the northern provinces, but has only recently received public attention. During this winter haze, the harmful particle pollution level is high throughout the day, including on weekends.

'Unfortunately, spraying water into the air does not appear to be effective, since the volume of water is minuscule compared to actual rain.

'With the start of the monsoon season, the weather in Bangkok will get better, and public attention on this issue will likely fade. However, without immediate policy measures, the problem will come back again next winter.'