Prime Minister Lee Hsien Loong said on Tuesday that the Government is monitoring the haze situation closely and will stay in touch with the Indonesian authorities.

He added that Minister for the Environment and Water Resources Vivian Balakrishnan and Foreign Minister K. Shanmugam have called their Indonesian counterparts to register Singapore's "serious concerns" about the situation and to offer help to fight the fires there.

Posting a Facebook photo of the haze taken at the Istana, Mr Lee said: "The city in the distance is barely visible. We are all affected by the haze. Farmers and plantation owners in Sumatra are burning crops to clear land in the dry season, and unfortunately the winds are blowing the smoke all the way to Singapore."

Earlier on Tuesday, Mr Shanmugam said on Facebook that he had spoken to Indonesian Foreign Minister Marty Natalegawa twice to stress the urgency of the situation, and reiterated Singapore's commitment to assist Indonesia with its fire-fighting efforts.

"He has discussed the issue with his Indonesian Cabinet colleagues to get the agencies to share information. This will help to better monitor the hotspots and land clearing activities," Mr Shanmugam added.

Dr Marty told The Straits Times that he hoped experts from Singapore and Indonesia's environment and forestry ministries could meet soon.

He said: "I had a good talk with Minister Shanmugam... and hopefully the experts can get together soon to compare notes on what's happening and where, and what's being done."

Dr Balakrishnan added on Facebook that he asked Minister Balthasar Kambuaya to name the companies responsible for the fires there causing the haze.

The Pollutant Standards Index hit 155 on Monday night but fell steadily throughout Tuesday before rising again. The reading at 9pm was 108. Air becomes unhealthy when the index crosses 100.