Haze returned to Singapore on Wednesday, with an air pollution index climbing into the " unhealthy" range.

The 3-hour pollutant standards index (PSI) hit 102 on Wednesday evening, according to the National Environment Agency. The " unhealthy" range of the PSI reading is from 101 to 200.

The National Environment Agency said in an advisory that the haziness was likely due to smoke haze from the surrounding region, blown in by prevailing winds.

The total number of hotspots detected in Sumatra on Wednesday was 23, while 139 hotspots were detected in Kalimantan.

"Smoke haze was visible over the southern half of Sumatra," the agency said. "Widespread thick smoke haze was observed in southern and western parts of Kalimantan. Some of the haze has spread to the southern parts of the South China Sea."

Occasional slight haziness may be experienced during the day on Thursday, it said.

The transboundary haze caused by the practice of slash and burn to clear land for agricultural purposes has been a headache for countries in the region. The PSI reading hit more than hazardous levels last year in Singapore and Malaysia, prompting calls for more efforts to reduce the number of hotspots. The transboundary haze is also a diplomatic challenge in the region.

The Indonesian parliament recently ratified the ASEAN ( Association of Southeast Asian Nations) Agreement on Transboundary Haze Pollution, a move welcomed by other countries in the region.

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