The brown smog rapidly moved over the city and the surrounding Klang Valley in the late afternoon, covering the Petronas Towers, once the world's tallest buildings, and permeating office buildings with the smell of smoke.

The Environment Department said over 500 fires in Indonesia's Riau and South Sumatra provinces, as well as fires in Kalimantan and Malaysia's Sarawak state had made conditions hazy.

Lee Heng Keng, deputy director general of the Environment Department, said that the ASEAN Specialised Meteorological Centre located in Singapore had detected the fires on Monday.

"There were over 500 hotspots in Sumatra yesterday so we suspect it could be coming from that side," Lee told AFP.

Although the smoke had appeared abruptly, Lee said the haze, carried by monsoon winds from the southwest had just "taken some time to cross over."

Initial reports said the smoke was from peat fires at a park reserve in Cyberjaya, an area south of the city near the administrative centre of Putrajaya.

However Lee said the fire was too small to cause the thick haze.

Meanwhile local news reports said visibility on a major highway southwest of the city was down to 150 metres (yards). Radio stations broadcast rush hour warnings for motorists to drive carefully through the haze.

Cars in the city centre were forced to put on their headlights in the middle of the afternoon, while pedestrians walked with their hands over their mouths.

Haze caused by fires in Indonesia and Malaysia is a common occurrence during hot, dry seasons.

In 1997 and 1998 choking haze caused by Indonesian forest fires enveloped parts of Southeast Asia, including Malaysia, for months.

They caused an estimated 9.3 billion dollars in economic losses to the region due to serious health problems, traffic hazards and flight disruptions.

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