Singapore endured its worst air pollution levels of the year, as monsoon season brought haze from forest fires in neighboring Indonesia.

The city-state's Pollutant Standards Index (PSI) rose by 20 points from Thursday night to Friday morning, ranging between 65-75 -- well above the normal range of 0 to 50, according to Singapore's National Environmental Agency (NEA).

The Singaporean government advised its citizens to avoid outdoor activities and prolonged exertion, attributing the haze to "an increase in hotspot activities observed over Sumatra" in the past week.

The agency added that winds may provide some relieve, but the hazy smog-like conditions could remain until next week.

By 4pm on Friday (local time), the PSI has dropped back to around 50.

To keep the public informed on the level of air quality, the NEA announced on Friday afternoon that it will provide hourly updates of the 3-hour PSI from 7am to 7pm daily.

Air pollution caused by Indonesian forest fires has also become a recurring problem in the South East Asian region, especially during this time of year when the southwest monsoon season brings periods of dry weather.

According to BBC, the two Southeast Asian countries have been working on measures to reduce the illegal practice of forest-burning in Indonesia, which sometimes occur when farmers try to illegally light fires to clear land for planting.

The Straits Times newspaper of Singapore reported that forest fires were likely concentrated in the provinces of Jambi and South Sumatra, located just south of Singapore.

Nevertheless, some activists say that officials have not been effective in enforcing the regulations.