Indonesia named eight Malaysian plantation companies on Monday that it wants to hold responsible for setting fires on Sumatra in a practice that is illegal, although slash-and-burn clearing still happens.

Smoke from the fires blanketed the Malaysian capital for nearly a week, threatening public health, disrupting transport and shutting down schools, until rain and breezes scattered the smog last Friday, carrying it north.

On Tuesday, Malaysia's Commodities Minister Peter Chin said the heads of plantation companies accused of setting some of the fires had told him they were not involved.

"I confirmed with the CEOs involved. They categorically denied carrying out any burning operations," Chin told Reuters.

The Malaysian firms practiced zero-burning techniques, he added. "In any case, their estates are already matured, so why should they clear land for plantation by burning?"

The plantation firms plan to resolve the issue by talking to their Indonesian counterparts and the authorities, Chin added.

Malaysia, fearing a return of the haze if the fires are not quickly put out, has sent a team of 100 firemen to help douse the flames. Australia plans to send a team of 12 bushfire experts.

Indonesian officials said the situation had improved.

"The situation is getting better. Hotspots are reduced. I don't know for sure how much," said Chaerul Zainal, chief of the environmental supervisory agency in Riau, where the bulk of the fires are concentrated.

In 1997, haze from mainly Indonesian fires blotted out skies across Southeast Asia. The fires are a perennial irritant, with Indonesia urged to act more quickly and Malaysian firms accused in turn of being part of the problem.

Malaysia complains Indonesia has yet to ratify a regional agreement aimed at controlling forest fires in Southeast Asia, while Indonesians blame Malaysian-owned palm-oil plantations both in Indonesia and in Malaysia for contributing to the haze.

Malaysia is the biggest producer of palm oil and, during drier weather at this time of year, plantation owners sometimes flout bans on open burning to clear land to plant new trees.