Michael Casey, Jakarta – Indonesia will prosecute 10 companies – eight of them Malaysian – for illegally lighting fires on Sumatra Island that have enveloped much of Malaysia with smoke.

The widespread haze, which appeared on Aug. 2, has forced the closure of schools and workplaces and dramatically increased respiratory ailments.

"The Environment Ministry has investigated those companies involved in slash-and-burn activities and we will prosecute them," said Indonesia's Forestry Minister Malam Kaban. The minister released the list of Malaysian firms but refused to divulge the Indonesian ones.

The haze is caused by fires on Indonesia's Sumatra island – across a narrow strip of sea from Malaysia – where farmers, plantation owners and miners clear land during dry weather. It blows annually across the strait to Malaysia, which has often complained that Indonesia does little to prevent it.

In Malaysia, authorities planned to crack down on local oil palm plantation owners who illegally burned agricultural waste, causing thick smoke to surround Kuala Lumpur International Airport on Sunday, a day after haze from the Indonesian fires had dissipated.

Conditions in Malaysia have improved since Friday due to a change in the wind direction and heavy rains, but five northern towns were still experiencing unhealthy air, according to Malaysia's Environment Department.

The worst haze crisis previously in the region was in 1997-98, and was also caused by Indonesian forest fires.