Singapore registered its worst level of air pollution for the year on Friday as smoke from forest fires in Indonesia blew over the city-state, triggering a health warning.

As of 8am (0000 GMT) Singapore's Pollutant Standards Index (PSI) ranged from 65-75 after a night of thick, choking haze over the island. The air began to clear later in the morning thanks to favourable winds.

A PSI reading of 65-75 is still in the moderate range. Anything above 100 is considered unhealthy.

"For the past one week, an increase in hotspot activities was observed over Sumatra," the National Environment Agency said in its latest update.

"The current prevailing winds blowing from the southwest or south have transported the haze from fires in southern Sumatra towards Singapore," referring to the Indonesian island.

The agency said hazy conditions could still recur and advised people with heart or lung disease as well as children and older adults to "avoid all physical activity outdoors."

"Everyone else should avoid prolonged or heavy exertion."

Singapore's Straits Times newspaper quoted a weather forecaster from West Kalimantan in Indonesia as saying that the haze in Singapore was likely from forest fires concentrated in the provinces of Jambi and South Sumatra.

Haze caused by the fires in Indonesia builds up during the dry season when farmers clean their land by burning, affecting tourism and contributing to health problems across the region.

Indonesia's government has outlawed land-clearing by fire but weak law enforcement means the ban is largely ignored.