Singapore woke up to clear blue skies due to favourable winds, but Malaysia was still being suffocated by smog from forest fires in Indonesia, where cloud-seeding flights have produced little rain.
In response to the severe haze that has hit the south of the country particularly hard, Malaysia has declared a state of emergency.
In the capital Kuala Lumpur, the pollution index was near the "very unhealthy" 200 level for the first time during the current outbreak.
Officials in Singapore, which bore the brunt of the smog last week, warned against complacency, saying the situation could deteriorate again if monsoon winds carrying smoke and particulates from Indonesia's Sumatra island changed direction.
Attempts by Indonesia to induce rain over the fires have had little success. Military planes are spraying water into the atmosphere to try to create clouds and rain to contain the fires.
"So far we had two sessions of cloud-seeding, one on Saturday evening and one yesterday evening. Some rain has fallen over Dumai city (in Riau province)," Indonesian disaster agency official Agus Wibowo told the AFP news agency in Jakarta.
"The cloud-seeding technology is meant to speed up rainfall, but with few clouds, there's little we can do. The rain was more like a drizzle."
Al Jazeera's Stephanie Scawen, reporting from Kuala Lampur, said that the haze in Malaysia was worse compared to that of Singapore.
"You can smell the smoke, it stings your eyes," she said.
In Malaysia, schools in Kuala Lumpur and several states were ordered to close and authorities advised parents to keep children indoors or make them wear face masks outside.
|Indonesia wildfires choke Singapore
Pollution levels in Malaysia's south eased on Monday but generally worsened elsewhere, with the city of Port Dickson, which lies on the Malacca Strait across from Sumatra, hitting the "hazardous" 335 level.
Conditions in densely populated Singapore first began to improve from "harmful" on Saturday and the pollutant index early on Monday was around 50 - within the "good" air-quality bracket.
"We must expect the haze to come back," Singapore's Prime Minister Lee Hsien Loong warned on his Facebook page.
"Look out for one another and help your neighbours, especially older Singaporeans and young kids."
Environment Minister Vivian Balakrishnan said on Sunday that "the improvement in the air quality is due to a change in the direction of the low-level winds over Singapore".
"However, we must remain prepared for further fluctuations depending on weather conditions," he added.