KUALA LUMPUR -- One of Malaysia's biggest marathons was cancelled on Saturday over fears the health of more than 30,000 runners was at risk from thick smoke caused by Indonesian forest fires that have sparked a regional environmental crisis.

Organisers of the Standard Chartered Kuala Lumpur Marathon set for Sunday said it would not be held due to worsening air quality in Kuala Lumpur, which was shrouded in an acrid grey haze from the slash-and-burn fires.

The haze has afflicted large swathes of Southeast Asia for weeks, sparking health alerts,school shutdowns and affecting flights.

Robert Field, a Columbia University scientist based at NASA's Goddard Institute for Space Studies, has been quoted by the space agency as saying that a possible longer dry season might make the 2015 haze crisis "the most severe on record".

Scientists predict the current crisis could surpass 1997 levels, when out-of-control fires sent pollution soaring to record highs in an environmental disaster that cost an estimated US$9.0 billion.

The fires on plantations and peatlands that are being illegally cleared by burning are located on Indonesia's huge islands of Sumatra and Borneo.

Close to half of Malaysia's 52 pollutant monitoring stations around the country, including those in the capital, registered "unhealthy" air quality on Saturday.

"The health and safety of all our runners remains our top priority," organisers of the marathon, with prize money at $120,000, said in a statement on their website.

Runners applauded the cancellation of Kuala Lumpur's 42.2-kilometre marathon, with some taking digs at Indonesia over the mess.

"This cancelation needs an apology from (the) Indonesian government as well," read one comment on Facebook.

Another social media remark read: "Wise decision by the organisers. No one wants to cancel a big event like this but the air quality is really bad. Safety of runners is of paramount importance."

The annual Kuala Lumpur race has gained popularity since it started in 2009 with more participants from overseas taking part over the years.