SINGAPORE - Air quality in the Republic has worsened, prompting authorities to write to Indonesia expressing "deep concern" over the situation.

A thin veil of grey hung over most areas of Singapore on Friday while the 24-hour Pollutant Standards Index (PSI) - a measure of air quality here - hovered between the high end of the moderate range and the low end of the unhealthy band from the early hours.

At 2pm, the 24-hour PSI across Singapore was in the range of 95 to 103. A reading above 100 refers to unhealthy air quality, in which prolonged or strenuous outdoor physical activity should be reduced.

At moderate levels between 50 and 100, normal activities can still be carried out.

The poor air quality has persisted in the Republic since Monday, when air quality crossed into the unhealthy band for the first time since April.

The deteriorating situation led Minister for the Environment and Water Resources Vivian Balakrishnan to write to Indonesia on Thursday expressing Singapore's "deep concern over the transboundary haze from Indonesia" that has affected the country.

Dr Balakrishnan said in a Facebook post that the haze then had reached unhealthy levels due to smoke from Kalimantan.

The haze was expected to fluctuate between moderate and mildy unhealthy levels due to dry weather in Kalimantan and wind direction, he added.

"I have written to the Indonesian Minister of Environment, expressing our concern with the deterioration. I also reiterated our offer of assistance to deal with the fires," Dr Balakrishnan wrote. "We hope they take urgent action on the ground."