Fires cloaked parts of Indonesia's Sumatra in thick smog Tuesday, forcing the cancellation of flights, two months after blazes on the island sparked Southeast Asia's worst haze crisis for years.

Two water-bombing helicopters were deployed to tackle the blazes on Sumatra, where haze generated by forest and peatland fires set to clear land for cultivation is an annual problem.

Some 488 fire hotspots had been detected by late Tuesday, according to the national disaster mitigation agency.

Flights to and from Pekanbaru city, the capital of Riau province, had been disrupted, with only one able to land in the past two days, agency spokesman Sutopo Purwo Nugroho said.

However, he added that neighbouring Singapore and Malaysia, badly hit by the June haze crisis, were "not affected at the moment because the wind is blowing northwest, so its only spreading over Riau".

The blazes in June on Sumatra left Singapore and Malaysia choking on the worst haze in more than a decade.

The air pollution scared off tourists, forced schools to close and caused a rise in respiratory illnesses.

The fires have been largely blamed on palm oil firms using the illegal but cheap method of burning vast tracts of rainforest and peatland to clear them for planting.