Malaysian authorities on Tuesday ordered school closures in Kuala Lumpur and neighbouring states as worsening haze from Indonesian forest and agricultural fires enveloped the capital in a smoky grey shroud.

Malaysia's education ministry ordered schools closed in Kuala Lumpur, three adjacent states and the nearby administrative capital of Putrajaya, where air pollution indices were in the upper range of "unhealthy" and nearing "very unhealthy".

More than half of the country's 52 monitoring stations registered "unhealthy" air on Tuesday.

Air quality in Malaysia and Singapore has steadily deteriorated over the past two weeks due to the smog from nearby regions of Indonesia, which on Monday declared a state of emergency in a particularly hard-hit province.

Smog-belching fires are an annual problem during the dry season in Indonesia, where vast tracts of land are cleared on the huge islands of Sumatra and Borneo using illegal slash-and-burn methods.

Indonesia has declared a state of emergency in Sumatra's Riau province.

Tens of thousands of people in smoke-choked regions of Sumatra and Borneo have fallen ill, while air travel there - as well as in parts of Malaysia - has been hit by sporadic flight delays or cancellations due to poor visibility.

Malaysian authorities in several areas have distributed free face masks, while the marine and aviation sectors have been advised to go on high alert due to the worsening visibility.

Authorities advised people with respiratory conditions to wear face masks outdoors and for all citizens to limit unnecessary outdoor activity.

Indonesia has come under pressure to control the annual smog outbreaks but joint regional efforts have failed to find a lasting solution.