The worst of the smog has shifted from Singapore to southernmost Malaysia, where noxious fumes have drifted across the sea this past week from Indonesia's Sumatra island.
The Malaysian government's index for air pollution reached a measurement of 746 early Sunday in the southern district of Muar. It was far above the threshold of 300 for hazardous air quality.
Authorities were issuing instructions for Muar's residents to remain indoors and for schools to close, Environment Minister G. Palanivel said in a statement on his Facebook page. The district has about 250,000 people, several of whom posted photographs on Twitter showing bridges and buildings enveloped in smog that slashed visibility to barely hundreds of meters (feet).
Air quality reached unhealthy levels in Kuala Lumpur, Malaysia's main city further north, where landmarks like the Petronas Twin Towers were obscured amid hot, humid conditions.
Malaysia's highest recording for the pollution index was 860 in 1997, one of the region's worst years for smog sparked by Indonesian blazes. Palanivel said officials might soon have to implement "cloud-seeding" operations meant to try to chemically induce rain.
Singapore reported pollution readings that fluctuated between "moderate" and "unhealthy" classifications Sunday, far below peak measurements recorded Friday. However, the city-state's officials have warned the respite, which has brought clear, blue skies to Singapore for the first time in nearly a week, might be only temporary because of current wind conditions.
This year's haze has been the worst in Singapore's history, forcing the military to suspend outdoor training, causing the cancellation of sports events and prompting the environment minister to travel to Indonesia to press for stronger action.
Jakarta has sent planes to help douse the fires in Sumatran peat swamp forests and sought to deflect criticism over its response to the environmental crisis. Some officials say Malaysian and Singaporean firms should also be held responsible because they have stakes in plantation land where the fires are raging.
Several of the companies have denied knowledge of fires in their official operating areas in Indonesia. In past years, similar blazes have been illegally set as a cheap method to clear land. Indonesian authorities say they are investigating whether anyone can be prosecuted for the current fires.