Indonesia is investigating 10 firms over worsening forest fires that have created a blanket of smog over Southeast Asia, threatening them with sanctions if they are found responsible, a government minister said on Tuesday.

The thick haze from Indonesia’s Sumatra and Kalimantan islands has forced the repeated cancellation of flights in the area and pushed air quality to unhealthy levels in neighboring Singapore and Malaysia.

Environment Minister Siti Nurbaya Bakar said companies could face sanctions if found violating their permits.

“While the legal process is going on, in parallel, there has to be a decision or act taken on the company’s permit,” the minister told reporters.

Sanctions range from a written warning to a fine to revoking a firm’s permit.

The minister named only one of the 10 firms under investigation, a small private company called Tempirai Palm Resources. It was not immediately possible to reach it for comment.

Indonesia has failed in previous attempts to stop the regional haze, with 2013 giving the worst pollution readings since 1997.

The heavy smoke from slash-and-burn clearances often comes from the islands of Sumatra and Kalimantan, where large forest concessions are used by pulp and paper and palm oil companies, some of which are listed in Singapore.

The companies blame smallholders for the fires, but they have been criticized by green groups for not doing enough to stop the haze or the rampant deforestation and destruction of carbon-rich peatlands in Indonesia.