Singapore Prime Minister Lawrence Wong has vowed to investigate firms responsible for fires that have shrouded the city-state in haze for weeks, warning that those found culpable could face legal action.

“We are looking into which companies may be responsible for the fires,” Wong said in a Facebook post late Thursday. “Where warranted, we will take action under Singapore’s Transboundary Haze Pollution Act, and also explore joint action with other countries.”

The city-state and neighboring Malaysia have been blanketed for weeks by toxic smog from a severe wildfire season in Indonesia that’s been exacerbated by a powerful El Nino weather pattern, bringing dry weather to the region. Governments are under increasing pressure to respond to what is the worst bout of haze since 2015.