Singapore has slammed 'shocking' statements from Indonesian officials over the haze crisis as it closed all schools and distributed protective face masks, AFP reported.

The city-state has been cloaked in smog blown in from tinder-dry Sumatra island for about three weeks, the worst such episode since mid-2013 in a crisis that grips the region nearly ever year during the burning-off season.

The closure of primary and secondary schools, as well as government-run kindergartens, is unprecedented, the Straits Times daily said as the air quality index hovered above 300, a level considered 'hazardous'.

As the pollution index rose, so did tempers, with Singapore Foreign Minister K. Shanmugam speaking out against some Indonesian figures who made light of the problem that has long strained relations among affected countries including Malaysia, AFP said further.

While Jakarta says it is taking steps to deal with the problem, 'at the same time, we are hearing some shocking statements made, at senior levels, from Indonesia, with a complete disregard for our people, and their own', Shanmugam said in a Facebook post late on Thursday.

'How is it possible for senior people in government to issue such statements, without any regard for their people, or ours, and without any embarrassment, or sense of responsibility?' he said.

"The Singapore government takes the matter seriously. We stand ready to assist Indonesia in combatting the fires. Last year, we introduced legislation to allow us to prosecute companies found to be causing or contributing to the haze. And we will not hesitate to take such actions against those errant companies" the Minister added.

Shanmugan did not identify the officials, but Indonesian Vice-President Jusuf Kalla has made waves in recent weeks by repeating comments he made in March that Indonesia's neighbours should be grateful for good air quality most of the year.

'For 11 months, they enjoyed nice air from Indonesia and they never thanked us,' he said at the time.

Singapore's Prime Minister Lee Hsien Loong said that offices will remain open but that face masks will be distributed at community centres across the island for needy and elderly citizens.

'There is no national shutdown of work, but employers should not compromise on the health and safety of their employees, especially those working outdoors,' he said in a Facebook post late Thursday.

The National Environment Agency said the haze was being blown in by prevailing winds from fires on Sumatra, where smallholders and agricultural companies use burning as a method to clear land for plantations during summer.

(With inputs from AFP)