Hunting is contributing to the haze that has become an annual hazard in several parts of Indochina during the hot, dry months from February to April, when farmers burn their agricultural waste as well as clear land by fire to prepare for the next crop.

According to Dr Nion Sirimongkonlertkun, a lecturer from the Rajamangala University of Technology Lanna who works closely with firefighters in the northern Thai province of Chiang Rai, some of the biggest fires in recent weeks have been caused by hunters setting forests alight to expose wildlife.

"When (the firefighters) go to check, they see people there waiting with a gun to kill animals," she told The Straits Times.

Nasa-based satellite imagery on Tuesday and yesterday showed several hot spots raging in Cambodia, Vietnam, northern Thailand, as well as in Myanmar and Laos near the Thai border.

Yesterday, the amount of air pollutants measuring 10 micrometers or less in diameter breached the safe level of 120 micrograms per cubic m in the Thai border provinces of Chiang Rai and Mae Hong Son.

Doctors say they usually see a jump in the number of patients with respiratory ailments during this time of the year.

Depending on the wind direction, the haze can reduce visibility to just a few hundred metres. This has forced airlines to cancel or divert some flights recently in Chiang Mai and Mae Hong Son.

The haze in Thailand's northern provinces can blow to other countries like Laos and Myanmar, said Mr Maytee Mahayosananta, who heads the government's Northern Meteorological Centre.

"But they won't go down to the central area or the south, as the wind during this season doesn't travel in this direction and is not that strong."

Thai officials have tried several methods to control the haze, including a ban on burning from mid-February to mid-April.

But locals say it has little effect in mountainous areas, where waste disposal transportation is both time-consuming and expensive, and hence there is little incentive to heed the ban.

In recent years, corn farms have been singled out for causing the haze. The hardy crop, which can be harvested in just four months, is a major component of animal feed.

Put on the defensive, agricultural conglomerate Charoen Pokphand earlier this week disclosed that it was setting up a social enterprise to steer farmers in Nan province away from corn, the Bangkok Post reported.