Indonesia Monday declared a state of emergency in a province choked with thick haze from forest fires, as fears mounted that worsening air quality could affect the upcoming Grand Prix in neighbouring Singapore.

The emergency announcement in Riau province on Sumatra island came as aircraft were deployed to water-bomb the raging blazes and conduct "cloud-seeding", which involves chemically inducing rain.

Smog-belching fires are an annual problem during the dry season in Indonesia. Vast tracts of land are cleared on Sumatra and the Indonesian part of Borneo island, using illegal slash-and-burn methods to make way for palm oil and pulp and paper plantations.

The blazes intensified in the past fortnight, sending smog over Sumatra and Borneo that has left tens of thousands ill, forced people to wear face masks and prompted the cancellation of flights and school closures.

Although there were more fires in other provinces, Riau has been one of the worst-hit as smog blows in from the surrounding areas.

Environment Minister Siti Nurbaya Bakar announced the state of emergency and said that more heath posts would be set up in Riau.

Arsyad Juliandi Rachman, Riau acting governor, said assistance would now be requested from the central government, state-run news agency Antara reported.

Air quality has also been worsening in Singapore just east of Sumatra and Malaysia. Authorities in Singapore said it was in the upper end of the "unhealthy" range late Monday, with smog shrouding the city-state and a smell of burning wood.

There were concerns about the potential impact on next weekend's Formula One, with organisers saying they were monitoring the haze.

"In the event that the haze caused visibility, public health or operational issues, Singapore GP would work closely with the relevant agencies before making any collective decisions regarding the event," a Singapore GP spokesman said in a statement.

Bakar agreed to share the names of companies suspected of causing the fires with Singapore during a phone conversation with Singapore Environment Minister Vivian Balakrishnan Monday afternoon, authorities in the city-state said.

Singapore has passed legislation allowing authorities to fine companies that cause or contribute to haze, regardless of whether they have an office in the city. But Indonesia has previously faced criticism for failing to hand over information.

Balakrishnan "expressed his concern over the serious haze situation" and reiterated Singapore's offer of help in fighting the fires, Singapore authorities said. Bakar said she would consult President Joko Widodo.

Haze levels have also been steadily rising in neighbouring Malaysia due to the Indonesian fires, with buildings more than a kilometre (0.6 miles) away in the capital Kuala Lumpur appearing as vague silhouettes.

Pressure to stop the annual outbreaks of smog has increased since 2013 when Southeast Asia suffered its worst air pollution crisis for more than a decade. But joint attempts by countries in the region to find a solution have moved slowly.

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