Singapore on Friday named four Indonesian companies that it says may have contributed to the smoke haze blanketing parts of Southeast Asia, adding that it would apply more pressure on palm oil and forestry companies responsible for forest fires.

Investigations aided by meteorological data and satellite imagery showed fires on Indonesian land concessions controlled by PT Rimba Hutani Mas, PT Sebangun Bumi Andalas Wood Industries, PT Bumi Sriwijaya Sentosa and PT Wachyuni Mandira, said the Ministry of the Environment and Water Resources on its website.

The ministry also said it has served Singapore-based Asia Pulp & Paper with a notice for information on measures taken by its subsidiaries and suppliers to put out fires on their Indonesian land concessions. The company did not respond to a request for comment sent after office hours.

A concentration of fires in parts of Indonesia’s Sumatra and Kalimantan in the past few weeks has driven the air quality of nearby cities to hazardous levels, data compiled by Global Forest Watch show. Last week, Indonesia set a 14-day deadline to extinguish fires in Riau and a 30-day target for South Sumatra and Jambi. The government has detained suspects in companies located in various parts of the country.

In Singapore, smoke from the fires caused the cancellation of some outdoor events. Primary and secondary schools were closed on Friday after the three-hour pollutant standards index reached the year’s high of 341 at a 5 a.m reading. The gauge stood at 154 as of 9 p.m., well into the "unhealthy" air quality level, deemed by the National Environment Agency.

Environment Minister Vivian Balakrishnan said the government is still gathering evidence into companies responsible for the fires and stressed that haze is a "man-made problem that should not be tolerated."

"Ultimately, errant companies must know that there is a price to be paid for damaging our health, environment and economy," he said in a statement on the website.

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