JAKARTA — Four people died on Indonesia’s Java island fighting the blazes that have been sending toxic smoke across southeast Asia, closing schools and fuelling a surge of respiratory complaints in the region, authorities said today.

The impact of the months-long fires has prompted Singapore’s leader to vow legal action against any companies responsible for the blazes that have made regional air quality among the world’s worst.

Singapore again topped IQAir’s live ranking of the world’s most polluted major cities today, with Kuala Lumpur second and Jakarta sixth.

“Where warranted, we will take action under Singapore’s Transboundary Haze Pollution Act, and also explore joint action with other countries,” said Singapore Prime Minister Lawrence Wong, without giving further details.

Singapore has no plantations or forests of its own, but several companies with concessions in Indonesia are reportedly based in the city-state.

Legal experts have cautioned that pursuing companies linked to the smoke is challenging, with investigations reliant on cooperation from foreign authorities.

Singapore enacted the Transboundary Haze Pollution Act in 2014, after a severe pollution episode in 2013. Another spell occurred in 2015.

The law allows authorities to target companies whose activities overseas cause or contribute to unhealthy haze levels in Singapore.

Establishing responsibility is challenging because it requires authorities to share maps pinpointing fire hotspots, with overlapping land claims further complicating matters.

Singapore’s investigations into four Indonesian companies over their alleged role in the 2015 haze crisis remain open, a case that strained relations with Jakarta at the time.

Other chances for legal action, including a possible case at The Hague-based International Court of Justice, were also low as it needed Indonesia’s consent for jurisdiction in the case.