Smoke from forest fires in Indonesia has grounded flights on the island of Sumatra and spread to affect several cities on neighbouring Malaysia.
Malaysians have been warned it could get worse
The smoky haze, an annual problem caused by the burning off of farm land and forest clearance, disrupted flights at the Sumatran city of Pekanbaru.
In Malaysia, the education minister said he would consider closing some schools if the air quality worsened.
A BBC reporter in Kuala Lumpur says it is the worst haze since 1997 and 1998.
The haze in those years caused serious health and economic problems across the region.
Singapore was warned to expect slightly hazy conditions in the coming days.
Officials blamed the latest haze on 300 areas of burning on Sumatra, where laws against the practise are often poorly enforced.
In Malaysia, the haze has cut visibility and caused minor breathing problems in the capital Kuala Lumpur and in Penang.
The BBC's Jonathan Kent in Kuala Lumpur says the city's landmark Petronas Twin Towers are slowly disappearing into a fog of pollution.
Officials said that with little rain expected, conditions could get worse.
Education Minister Hishammuddin Hussein has said he is considering banning all sports and outdoor activities for children.