The city-state has beefed up measures to combat the months-long fires and had vowed legal action against companies responsible for the blazes that have made regional air quality among the world's worst.

"Our investigation so far suggests that some of the haze experienced in Singapore originated from fires on the lands of seven companies in Indonesia," Sustainability and Environment Minister Grace Fu said.

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Singapore will send the "preventative notices" to the companies, which are based on Indonesia's Sumatra island to the west.

The notices tell the companies to deploy fire fighting teams, stop burning or starting new fires and submit their action plans to Singapore's National Environmental Agency.

Asked what steps Singapore could take to ensure the companies complied, Fu said measures like "fines or charges" were "all within our realm of consideration".

The companies named by the Singaporean government are mainly pulpwood and timber suppliers in Sumatra and include Bumi Andalas Permai, Bumi Mekar Hijau and SBA Wood Industries.

Read also: Local, foreign firms own largest burned concessions in Kalimantan

Authorities said on Friday that four people have died fighting the blazes on Indonesia's Java island.

Two others suffered serious burns and were receiving further medical treatment, according to Indonesian authorities.

The blazes in Indonesia have closed schools and fueled a surge of respiratory complaints and growing frustration about the inability to contain the situation.

"We should be prepared for the haze to persist for some time," Singapore's Fu said, adding that "the situation is fluid".

"Depending on the weather, we can expect air quality to fluctuate, including the possibility of reaching hazardous levels," Fu said.

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Singapore topped IQAir's live ranking of the world's most polluted major cities on Friday afternoon, with Kuala Lumpur second and Jakarta tenth.

Legal experts have cautioned that pursuing companies linked to the smoke is challenging, with investigations reliant on cooperation from foreign authorities.

Singapore has no plantations or forests of its own, but several companies with concessions in Indonesia are reportedly based in the city-state.

Singapore's investigations into four Indonesian companies over their alleged role in the 2015 haze crisis remain open, a case that strained relations with Jakarta at the time.

Indonesia experiences wildfires regularly during its dry season, but this year the blazes have been especially severe because a strong El Nino weather pattern has brought drought.

While drought contributes to the fires, experts say deforestation and drying of peatlands has also damaged previously fire-resistant landscapes.

"We cannot control the weather or the winds, nor can we directly control what happens beyond our borders," said Singapore leader Lawrence Wong.

On Friday afternoon, the city-state's air pollution lay at around 230 on IQAir's Air Quality Index, considered within "very unhealthy levels".

Under the Swiss-based company's system, readings of 151-200 are considered unhealthy, 201-300 very unhealthy and above 300 hazardous.

Meanwhile, tourists in Singapore on Friday told AFP they experienced heavy air pollution.

"This is fairly thick, I don't know what the number is on the (air quality) index, but from my experiences, this is fairly heavy," said Canadian tourist Ron Chambers.

He added that the haze was affecting sight-seeing plans.

"I don't think there will be much of a view, so we're sticking to ground-level activities," he told AFP.

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