Singapore urged Indonesia to take “urgent measures” on Monday as severe air pollution from a rash of forest fires on Sumatra island choked the densely populated city-state.

Singapore's skyscrapers including the famous Marina Bay Sands casino towers were shrouded in haze and the acrid smell of burnt wood pervaded the central business district.

Parts of neighbouring Malaysia were also suffering from the smoky haze, a recurring problem Southeast Asian governments have failed to solve.

Singapore's Pollutant Standards Index soared to 111 by late afternoon on Monday, well past the officially designated “unhealthy” threshold of 100.AFP

Why is this happening?

The problem occurs in the dry season as a result of forest fires in the sprawling Indonesian archipelago, some of them deliberately started to clear land for cultivation.

Singapore's National Environment Agency said 138 “hotspots” indicating fires were detected on Sumatra on Sunday, and prevailing winds carried smoke over to Singapore.