Singapore is refusing to back down in its pursuit of those responsible for haze-belching forest fires in Southeast Asia last year, despite struggling to bring the perpetrators before the courts and drawing a sharp rebuke from neighbouring Indonesia.

Forest fires are part of an annual dry-season problem in Indonesia, started illegally to quickly and cheaply clear land for cultivation -- particularly for palm oil and pulpwood.

But last year's haze outbreak was among the worst in memory, shrouding Malaysia, Singapore, and parts of Thailand in acrid smoke and forcing school closures as pollution reached hazardous levels and thousands fell sick across the region.

Singapore has served notice to six Indonesian companies it believes may have cleared land by burning but could target others as investigations continue, according to Singapore's ambassador to Indonesia Anil Kumar Nayar.

"We are going after, to put it starkly, the bad guys that are causing this problem," he told AFP in an interview last week.

However, the city-state's efforts to punish Indonesian companies under its own anti-haze law have become a flashpoint with Jakarta.

Singapore argues that international rules allow states to take action -- even if harm is being caused by activities outside its jurisdiction -- but Jakarta has questioned how Singapore could pursue Indonesian citizens for prosecution, especially in the absence of a ratified extradition treaty between the neighbours.

The latest sabre-rattling came after Singapore issued a court warrant in May to detain a director of an Indonesian company linked to the haze while he was in the city-state.

Afterwards, Indonesia's Environment Minister Siti Nurbaya Bakar said that she would be reviewing her ministry's cooperation with Singapore on environmental issues.

"Singapore cannot step further into Indonesia's legal domain," Bakar told reporters in June. Her spokesman declined to comment further on the matter when contacted.

Nayar reiterated that Singapore wasn't crossing any line pursuing these companies and was within its rights to enforce its law.

"We are not doing something that is extraordinary. It is not targeting any country, or anybody's sovereignty," he said.

The law threatens local and foreign firms with fines of up to $100,000 Singaporean dollars (US$74,000) for every day Singapore endures unhealthy haze pollution.

So far just two of the companies have responded to the court order, Nayar said, without naming specific firms.

- Slow progress -

Singapore has repeatedly asked Indonesia for details about companies -- such as maps showing who owns what concessions -- but says Jakarta has not provided any information.

Singapore would "continue to press", Nayar said, but added the evidence needed to prosecute these companies could be found by other means.

"We could go that way as well, but at the end of the day this is part of a legal process. We want to be working with the Indonesian government," he said.

One of Indonesia's main arguments is that a regional approach to solving the haze crisis would be more effective than individual action.

"They (Singapore) know our view on this, on how we can best address this issue of haze through the ASEAN mechanism," ministry spokesman Arrmanatha Nasir told AFP.

The Association of Southeast Asian Nations has an agreement to create a haze-free region by 2020, though it took 14 years to be fully ratified.

Nayar says regional progress on curbing haze has been slow.

Fellow ASEAN member Malaysia, which also suffers during the haze outbreaks, has expressed interest in adopting its own law similar to Singapore's to pursue errant companies.

Jakarta has promised tougher action in the wake of last year's haze disaster, which turned skies yellow in Indonesia's part of Borneo island and dealt the economy a $16 billion blow.

The government announced in May it would no longer grant new land for palm oil plantations, and established a new agency to restore millions of hectares of carbon-rich peatlands susceptible to fires.