The Pollutant Standards Index (PSI) hit 401 at 12:00 on Friday (04:00 GMT) - the highest in Singapore's history.
The index also reached 400 in one part of Indonesia, which is readying helicopters and cloud-seeding equipment in an effort to tackle the fires.
Indonesia has said it is unfair to blame it solely for the forest fires.
A senior official in the Indonesian president's office said fires had been spotted on land owned by 32 companies in the region, some of them based in Malaysia and Singapore.