Singapore’s Prime Minister has warned that a haze blanketing Singapore from fires in Indonesia could persist for weeks or even longer.
Lee Hsien Loong also said there could be consequences if Singapore-linked companies were found responsible for the burning.
The illegal burning of forests and other land on Indonesia's Sumatra island, to the west of Singapore, to clear space for palm oil plantations is a chronic problem.
It is particularly worse during the June to September dry season.
Singapore has provided satellite hotspot data and imagery to Indonesia to help identify those responsible, the prime minister said.
"Some Indonesian officials have suggested that these errant companies may be linked to Singapore and Malaysia," Mr Lee said.
"If any Singapore companies are involved, or companies which are present in Singapore are involved, we will take it up with them."
Singapore's pollution standards index (PSI) has soared to a new high of 371, indicating air quality was "hazardous".
The city-state, which prides itself on its clean environment and usually enjoys clear skies, saw its air quality deteriorate to unhealthy levels on Monday.
The top PSI readings in Singapore over the past two days have exceeded the peak of 226 reached in 1997 when smog from Indonesian fires disrupted shipping and air travel across southeast Asia.