BANGKOK (AFP): Forest fires in Indonesia and elsewhere in Southeast Asia are being deliberately lit by agricultural firms, causing serious health and environmental damage, the UN's food and agriculture body said on Wednesday.

The Food and Agriculture Organization (FAO) called on Asian nations to enforce bans on open burning to prevent the annual haze crisis which last month sent a layer of smoke and dust over parts of Malaysia and Thailand.

"Most of these fires are intentional and planned by agro-industrial companies to clear forests for agricultural land use," FAO forest fire expert Mike Jurvelius said in a statement.

"Using fire to clear forests is prohibited in most of the Southeast Asian countries and the ban should urgently be enforced," he said, adding that unwanted plant material should be disposed of in other ways.

Malaysia and Indonesia have pledged to take action against plantation firms in both countries responsible for slash-and-burn farming methods which contributed to the haze.

Malaysia was worst hit by the crisis, which sent air pollution to extremely hazardous levels and forced schools and an airport to close.

"The environmental impacts are tremendous on the forests itself, as well as smoke affecting human health," FAO senior forestry officer Patrick Durst said.

Durst said the fires had become more serious over the past 15 years because of the El Nino weather pattern coupled with the opening up of forests by humans, which was drying out the habitat and making it more combustible.

The FAO said fire was a cheap and easy way to convert forest to agricultural land, but it often took place in regions with peat soil where fires can burn as much as 20 metres (65 feet) below ground.

"These fires are almost impossible to extinguish, regardless of how many airplanes or helicopters are used," it said.

"So long as people do not understand the dangers of using fires for land clearance on peat soils, the fight against forest fires will be very costly and have only limited success."