Haze blanketed parts of Malaysia on Monday, weeks after the region suffered its worst pollution from forest fires in Indonesia in more than a decade.
Three areas—two in southern Malacca state and a third in Selangor near the capital Kuala Lumpur—recorded "unhealthy" air quality with readings above 100, according to the Department of Environment.
In Malaysia, Air Pollutant Index readings of 100-200 are branded "unhealthy". Readings in much of the rest of the country were below 100 at "moderate" levels.
A department official said the haze—caused by blazes on Indonesia's Sumatra island due to the slash and burn method of land clearance for cultivation—was expected to continue for two to three days before rain would bring relief.
Malaysia and neighbouring Singapore last month choked on the worst haze in more than a decade, with readings of more than 300 marking "hazardous" air quality in some parts of Malaysia.
The pollution scared off tourists, forced schools to close and caused a rise in respiratory illnesses.
Skies were clear in Singapore on Monday, with the country's Pollutant Standards Index reading within the "good" band, the National Environment Agency website said.
Environment Minister Vivian Balakrishnan said although Singapore was spared because of the current wind direction, the country had already contacted Indonesian authorities and the ASEAN coordinating centre "to register our major concerns".
He added the government would work with NGOs to identify the companies with concession areas affected by fire.
"We need these companies and the Indonesian authorities to do their part to extinguish these fires... We all need to be vigilant," he said in a statement.
After meeting Balakrishnan and other counterparts to discuss the problem last week, Indonesia's environment minister Balthasar Kambuaya said that the country hoped to ratify a regional treaty to fight the smog by early next year.
Indonesia is the only member of the Association of Southeast Asian Nations which has still not ratified its Agreement on Transboundary Haze Pollution, brokered in 2002.
The treaty aims to stop the cross-border smog by requiring parties to prevent burning, monitor prevention efforts, exchange information and provide mutual help.
Explore further: Tropical storm batters southern Mexico coast, kills six