Soldiers battle grass fires in northern Thailand which have made the region the worst in the world for air quality.

Hundreds of officers from Royal Thai Army joined fire crews as they tried to extinguish vegetation fires in Chaing Mai province yesterday (March 15).

The smoke has made the popular tourist city the worst in the world for air pollution, recording PM2.5 readings of 282 - almost six times the safe level of 50.

Soldiers searching for fire in the mountainous region have also been given the authority to detain anyone they find starting blazes - a common way for farmers to burn waste crops.

Authorities said the fires cause small particles which become trapped in the air by a combination of low pressure, dry weather and a lack of wind. The pollution then lingers in the air over towns and cities.

Chiang Mai governor Charoenrit Sa-nguansat warned residents against outdoor activities and urged them to wear protective masks when going outside.

He said the poor air quality has been made worse by agricultural burning in rural Thailand and neighboring Myanmar, Cambodia, and Laos, which created the choking haze across the northern region.