It has been one month since out-of-control forest fires in Indonesia blanketed large parts of South-east Asia in a thick smoky haze, and environmentalists say the smog's persistence is evidence that Indonesia is unable to do much to deal with the region's biggest environmental threat.

A thick layer of smog covering parts of Indonesia, Malaysia, Brunei and Singapore, caused by land clearing forest fires on a massive scale, has been a yearly occurrence since the mid-1990s.

The last big blaze was in 1997 to 1998 and resulted in the destruction of about 10 million hectares of natural forest and the exposure of more than 20 million people to dangerous pollutants, according to the United Nations Environment Programme.

This year, satellite photographs over the region show around 20,000 hotspots where forest fires are burning in Indonesia's Kalimantan on the island of Borneo, where the haze is worst, and Riau province in southern Sumatra, according to Indonesia's environment ministry.

After easing a bit two weeks ago, the haze thickened in Kalimantan last Monday and Tuesday, as the fires are aided by the dry season and winds. Sjahrani Sjahrin, head of the Central Kalimantan Environment Control and Managing Agency, said: ''The only way to extinguish the hotspots is heavy rain.''