More than 300 schools and kindergartens in Malaysia's southern state of Johor have been ordered to close on Sunday after the haze reached "very unhealthy" levels in three districts.

The Air Pollutant Index (API) in Tangkak, the worst-hit district, hit 208 at 5pm.

An API reading of between 0 and 50 is good; 51 to 100 (moderate); 101 to 200 (unhealthy); 201 to 300 (very unhealthy); and 301 and above is considered hazardous.

Malaysia closed hundreds of schools and sent half a million face masks to Sarawak earlier this week, after the smoke built up to unhealthy levels.

Illegal fires to clear land for farming are raging on Indonesia's Sumatra and Borneo islands, with firefighters battling round the clock through charred forests. Water-bombing helicopters have also been deployed to douse the flames.

The Malaysian Health Department director-general Noor Hisham Abdullah advised Malaysians to stay indoors when the API readings hit 100.