Pollution from the fires normally worsens during the day, but by 8:00 am Friday (0000 GMT), the three-hour average pollution index had already reached 103, according to the National Environment Agency's website.

Any reading between 101 and 200 is considered unhealthy and an advisory remained in effect urging people with heart or respiratory ailments to reduce physical exertion and outdoor activity.

The fog-like haze left the morning sun a faint orange disc in the sky, before the pollution index dropped back slightly to 92 by early afternoon.

Singapore's air quality has reached the unhealthy range for most days since last weekend, as the haze lingered in the city-state normally known for its cleanliness and green environment.

On Thursday, Malaysia's capital, Kuala Lumpur, was also shrouded in "unhealthy" levels of haze for a second straight day as smoke continued to drift from the Indonesian fires.

Indonesia has come under increasing pressure to act to douse blazes set by large plantations and farmers. The government has outlawed land-clearing by fire, but weak enforcement means the ban is largely ignored.

Indonesia's annual burn-off causes a haze that typically smothers parts of Malaysia, Singapore and Thailand, as well as Indonesia itself.

In an editorial Friday, Singapore's The Straits Times said that although 16 companies are reportedly under investigation over the burning, neighbouring countries "want to be sure this is not a PR exercise undertaken to mollify angry opinion."