Singapore is sending legal notices to seven companies whose land in Indonesia was linked to fires behind some of the haze blanketing the city, Minister for Sustainability and the Environment Grace Fu said Friday, The Straits Times and CNA reported. The National Environment Agency has issued the preventive measure notices under Singapore's Transboundary Haze Pollution Act, which targets those who cause or condone fires, The Straits Times reported.

The notices tell the companies to deploy firefighters to put out or stop the spread of fires on their land, to stop or not start any burning, and to send the agency plans to put out the fires and prevent them from recurring, The Straits Times reported. Three of the firms, Bumi Andalas Permai, Bumi Mekar Hijau and SBA Andalas Wood Industries, received similar notices during the 2015 haze crisis, and those cases remain open, The Straits Times reported. The seven companies are in Sumatra, Fu said, according to The Straits Times.

Fu said the government is keeping its options open and is considering other steps, such as fines and charges, The Straits Times reported. "It is not easy, I will be the first to admit, but we must try," she said, according to CNA. The law was first used in 2015, when notices went to six Indonesian companies, and no fines have been imposed yet, The Straits Times reported.

Legal experts said action could carry diplomatic costs. When Singapore tried to use the law in 2015, "there were cries that Singapore was infringing upon Indonesia's sovereignty," Associate Professor Jolene Lin of the National University of Singapore said, according to CNA. Proving that a company's conduct caused haze in Singapore can be hard, Singapore Management University law professor Michelle Lim said, according to CNA.

Frustration with the haze is growing across the region, where it has closed schools and drifted as far as the Philippines and Vietnam, The Guardian reported. Prime Minister Lawrence Wong said Thursday that the government was "looking into which companies may be responsible for the fires," The Guardian reported. The head of Indonesia's government communication agency, Syahganda Nainggolan, pushed back at the criticism, saying "the oxygen in Singapore and Malaysia comes from our forests," The Guardian reported.

What we know: Singapore has issued haze-law notices to seven companies tied to fires in Indonesia and says fines or charges remain possible.
What we don't: Whether the companies will comply, or whether Singapore will move on to fines or charges.