An Indonesian province at the heart of a Southeast Asian smog crisis last year has declared a state of emergency after being blanketed in thick haze from forest fires, officials said Thursday.

Thousands have fallen ill, transport has been disrupted and schools closed after days of fires in Riau province on Sumatra island, where blazes are deliberately lit every year to clear land for palm oil and wood pulp plantations.

More than two dozen people suspected of starting fires in rainforest and peatland have so far been arrested, said national disaster agency spokesman Sutopo Purwo Nugroho.

Haze from fires on Sumatra is an annual problem in Southeast Asia, but last June Singapore and Malaysia were cloaked in the worst smog for more than a decade.

While some haze was detected in the two neighbouring countries in recent days, the air quality was mostly good.

Declaring an emergency allows Riau to seek help in tackling the blazes from the central government, and Nugroho said aircraft were preparing to drop water on fires and carry out "cloud-seeding" to chemically induce rain.

"The disaster agency is preparing aeroplanes and helicopters to carry out water-bombing of the fires," he said.

More than 25,000 people have fallen ill in recent days due to high air pollution, with most suffering respiratory tract infections, said Riau disaster chief Said Saqlul Amri.

The emergency status means health centres must see patients free of charge, he said.

Local media reported that some flights to Riau had been cancelled this week while others were diverted, and schools were closed in some parts of the province.

Last year's fires strained ties between Indonesia and its neighbours, prompting an apology from Indonesian President Susilo Bambang Yudhoyono to Malaysia and Singapore at the height of the crisis.

Indonesia is the world's biggest producer of palm oil -- which is found in everyday grocery items, such as shampoo and shaving gel -- and Riau is the country's main palm oil hub.