A thick haze shrouding Malaysia's tourist island of Penang and caused by fires in neighbouring Indonesia has spread to the mainland, meteorological officials said.

Air quality levels in several towns on mainland Malaysia, across from Penang and extending into northern Perak state, were officially unhealthy, a meteorological official in the Kuala Lumpur bureau told AFP.

An unhealthy reading means the air is likely to irritate eyes or cause sneezing and coughing, and may worsen chronic heart or lung ailments.

Visibility in Prai township was down to three kilometres (two miles) from a normal 10 kilometres, the department said on its website.

The deterioration in air quality was mainly due to the increase of "hot spots" or burning areas on the Indonesian island of Sumatra, the meteorological authority said. Department of environment director general Rosnani Ibarahim said about 500 hot spots were raging in Sumatra on Sunday and it was difficult to predict how long the haze would persist.

The haze was being blown over Malaysia by monsoon winds coming from the southwest, she said.

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.