Singapore has singled out seven companies it claims were linked to wildfires in Indonesia that have shrouded the city-state in smog, intensifying scrutiny of businesses connected to the blazes.

The National Environment Agency sent notices to the companies, demanding they stop fires from spreading and provide plans outlining the steps they are taking to control the situation, according to a statement on Friday. The agency said it relied on maps, meteorological data and satellite imagery to pinpoint the firms.

Among those given notices were PT Bumi Andalas Permai and PT Bumi Mekar Hijau, which run plantations in South Sumatra. Both are listed as suppliers to APP Group, one of the industry's largest players.

APP said in a statement on Saturday that all of its "suppliers adhere to a zero-burning policy." Bumi Mekar Hijau and Bumi Andalas Permai did not respond to phone calls requesting comment.

Singapore and Malaysia have been blanketed in toxic smog from wildfires in Indonesia, with dry conditions linked to a powerful El Nino worsening the fires. Singapore Prime Minister Lawrence Wong vowed to investigate companies responsible for the blazes and warned that those found culpable could face legal action.

Separately, Singapore's stock exchange questioned listed companies with agricultural and plantation operations in Indonesia about the forest and peatland fires, according to a statement from Singapore Exchange Regulation. Kencana Agri Ltd. and Bumitama Agri Ltd. told the exchange that blazes had spread into concessions they manage and that they were working to extinguish them.

Wilmar International Ltd. said it had put out fires in small areas of some concessions, saying they originated outside its estates. Golden Agri-Resources Ltd. said it wasn't aware of any evidence that its operations had caused or contributed to the haze.

Fires caused by slash-and-burn techniques used to clear farmland have long plagued Indonesia during its dry months. This year's particularly severe season has been exacerbated by El Nino, which has brought drier conditions to the region.

Singapore enacted its transboundary haze law in 2014, allowing authorities to fine companies as much as S$100,000 ($78,000) per day of pollution. The city-state sought to enforce it in 2015, but the environment ministry said at the time it was having difficulty getting information from its Indonesian counterpart on firms suspected of illegal burning. A decade on, four companies continue to be under investigation.

The law targets entities responsible for causing or condoning fires when resulting smog keeps the Pollutant Standards Index at unhealthy levels for at least 24 hours.

The 24-hour PSI ranged from 94 to 175 across the island as of 7 p.m. local time on Friday. Air quality is considered "very unhealthy" when levels are 201 and above. A reading higher than 300 is considered "hazardous" and can trigger school closures.

Sustainability and the Environment Minister Grace Fu said air quality is expected to fluctuate, with the PSI potentially reaching hazardous levels. Conditions will likely improve when the inter-monsoon period begins in three to four weeks, she said.