The Manpower Ministry may issue a stop-work order and a decision would have to be taken on whether to close schools and childcare centres if the haze situation worsens severely, said Environment and Water Resources Minister Vivian Balakrishnan in response to questions from reporters.

The Manpower Ministry will provide an update to employers on Thursday about what to do, while the Ministry of Health has also alerted hospitals to cope with a potential increase in patients with respiratory problems, he said at a press conference at 11.30pm on Wednesday.

The Government has also convened a task force - comprising 23 agencies - to coordinate plans to reduce the haze's impact on people.

Dr Balakrishnan was speaking after the Pollutant Standards Index (PSI) soared to 321 at 10pm, far exceeding the previous worst reading of 226 in 1997. Air becomes "hazardous" when the index crosses 300. The index dropped to 218 as of midnight.

Describing the haze situation as "unprecedented", Dr Balakrishnan said the National Environment Agency's (NEA) chief executive, Mr Andrew Tan, will lead a delegation to Indonesia on Thursday to an emergency haze meeting convened by the Indonesian Foreign Ministry.

The authorities here had expected the haze situation to worsen overnight, with 173 hotspots detected on Wednesday, said the minister. Dry weather conditions and prevailing winds blowing haze into Singapore are expected to persist for next few days, he added.

Conditions are not suitable for cloud seeding now, but the ministry will keep the option open, he pointed out.

"The key message for tonight is that we have to change our daily routine in the short term," he said.

Children, the elderly and people with existing diseases should stay indoors, keep windows and doors closed and avoid physical exertion if the PSI crosses 300, according to the NEA's health advisory. Everyone else should also avoid all unnecessary outdoor activity.