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

The city-state's Pollutant Standards Index, or PSI, rose by 20 points from Thursday night to Friday morning, ranging between 65 and 75 -- well above the normal range from 0 to 50 -- according to Singapore's National Environmental Agency, or 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 relief, but the hazy, smoglike conditions could remain until next week.

On Friday by 4 p.m. local time, the PSI had dropped back to about 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 three-hour PSI daily from 7 a.m. to 7 p.m. local time.

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

According to BBC News, 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.

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