A Malaysian company is among more than 20 firms under investigation by Indonesian authorities in connection with forest fires that have caused a haze to engulf large parts of South-East Asia, an Indonesian minister said yesterday.

The worsening smog across northern Indonesia, neighbouring Singapore and parts of Malaysia forced some schools to close and airlines to delay flights this week, while Indonesia ordered a crackdown on lighting fires to clear forested land.

"There is one (company) from Malaysia that is among those being investigated," Environment and Forestry Minister Siti Nurbaya told reporters, without elaborating. "We are still checking if there are any from Singapore."

Police in Riau, at the heart of the haze, said an official overseeing operations at Indonesian palm oil company Langgam Inti Hibrido had been named a suspect for starting fires on the company's land. It was not possible to reach the company outside of business hours.

South-East Asia has suffered for years from annual bouts of smog caused by slash-and-burn practices in Indonesia's Sumatra and Kalimantan islands, but governments in the region have failed to address the problem.

The fires have been exacerbated this year by the effects of the El Nino weather phenomenon, as a prolonged dry season in Indonesia has parched the topsoil, fuelling the flames.

This year's haze had already caused trillions of rupiah in losses to the Indonesian economy, with further losses now expected, added Nurbaya.

Authorities have so far declined to name any other suspected perpetrators but are set to announce this week the names of three or four companies that are due to face sanctions including possibly having their land permits revoked.

President Joko Widodo instructed security forces late on Monday to accelerate efforts to extinguish the fires and revoke land permits from companies found responsible.

Nearly 3,000 military and police personnel, 17 helicopters and four cloud-seeding aircraft have been deployed to fight the fires, according to the country's disaster management agency.

- Reuters