SINGAPORE - Singapore is experiencing "deteriorating hazy conditions" due to "an incoming haze cloud from Sumatra" Environment Minister Vivian Balakrishnan said in a Facebook post on Sunday.

As of 7pm, the 3-hour PSI stood at 89, which is in the moderate level. The forecast for the 24-hour PSI over the following six hours was in "the high end of the moderate range or even low unhealthy level", the minister added.

People posted pictures on Twitter of the worsening air quality with one user, "lucyda" complaning of a "burning wood smell".

A spokesman for the National Environment Agency (NEA) said: "Air quality is slightly elevated and there are reports of burning smells in the central and eastern regions of Singapore.

"NEA is monitoring the situation closely and will provide subsequent updates should the situation change."

The PSI measures soot, dust particles, sulphur dioxide, carbon monoxide, nitrogen dioxide and ground level ozone in the air. When the reading is above 100, it is recommended that people should cut down on prolonged or strenuous outdoor physical activity.

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