Officials in Singapore are considering whether to take legal action against two companies in connection with the severe smog that has clouded the city following forest fires in Indonesia.

“The majority of hotspots in Riau are inside APRIL and Sinar Mas concessions,” senior presidential aide Kuntoro Mangkusubroto told Reuters on Friday.

The companies Asia Pacific Resources International (APRIL) and Sinar Mas own land on the Sumatra Island and have head-quarters based in Singapore, but their owners are Indonesian.

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An APRIL statement said it and third-party suppliers had a “strict no-burn policy” for all concessions in Indonesia.

An analysis of satellite maps and government data by Reuters and the think-tank World Resources Institute revealed spot fires on land licensed to Singapore-listed First Resources Ltd and Indonesia's Provident Agro. The analysis did not show the cause of the fires or suggest who was at fault.

A spokeswoman for Golden Agri Resources, SMART's Singapore-listed parent, said it knew of no hotspots on its concessions.

Hadi Daryanto, the general secretary of Indonesia's Forestry Ministry, said a team led by Forestry Minister Zulkifli Hasan was now in Riau province on Sumatra “to find out the exact locations of the hotspots”.

“But we have to be very careful in any legal action,” he said. “We have to really find out what happened, why the fires happened and so on. This could be due to negligence, too.”

Singapore's pollution index has climbed back to “hazardous” levels and air quality deteriorated in the Malaysian capital on Saturday as Indonesia came under heavy pressure to bring fires from slash-and-burn land clearing under control.

Indonesia has deployed military planes to fight the blazes on Sumatra island from illegal burning that typically takes place in the June to September dry season to clear space for palm oil plantations. The fires are unusually widespread this year and the smog is the heaviest in Singapore's history.

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Companies behind the fires would find no refuge in Singapore, Foreign Affairs Minister K. Shanmugam said on Saturday, even as he acknowledged limits in international law to deal with firms that operate outside the city-state.

“We will do everything we can,” he told a news conference.

Air pollution has long been a serious problem in many Asian cities such as New Delhi, Beijing and Hong Kong but it is only an issue in Singapore when the smoke blows in from Sumatra.

According to one method of measuring pollution, the one authorities use in China's capital, Singapore's air was much worse than Beijing's on Friday, according to state agency data.

The cost of the current haze for Singapore could be hundreds of millions of dollars, brokerage CLSA said in a report.

It said that in 2006, when the pollution index reached 150, it was estimated the haze cost $50 million and in 1997 it was $300 million. CLSA said the 1997 and 2006 figures seemed low when considering the direct and indirect cost of prolonged haze.

Additional reporting by Reuters