Indonesia Tuesday deployed an extra 1,600 military personnel to fight forest and farm fires that have cast a thick haze over the region, closing schools in Malaysia and shrouding Singapore in smog just as it prepares to host the glitzy Formula One race.

President Joko Widodo ordered the military ramp-up on Sumatra after authorities declared a state of emergency in the island's hard-hit Riau province Monday.

The personnel were dispatched to Riau and South Sumatra provinces to help local authorities fight fires, joining over 1,000 soldiers sent to the area last week, Indonesia's disaster agency said.

Tens of thousands of people in smoke-choked regions of Indonesia have fallen ill, while air travel there -- as well as in parts of Malaysia -- has been hit by sporadic flight delays or cancellations due to poor visibility.

Malaysian authorities ordered the closure of more than 2,000 schools, affecting 1.5 million students, while the haze reportedly forced Prime Minister Najib Razak to scrap a planned helicopter ride and take a 90-minute road journey instead.

Najib had been due to travel by helicopter from Kuching, capital of the Malaysian state of Sarawak, to the town of Sematan 100 kilometres (62 miles) away, state-run Bernama news agency said.

But he instead had to travel by car to an event held to officiate the construction of a highway, it said.

Fears are mounting that the smog enveloping regional financial hub Singapore, where air quality remained at unhealthy levels, could affect this weekend's Grand Prix.

Organisers of Formula One's only night race, which sees cars speed along a brightly illuminated track alongside landmarks and is coupled with pop concerts, have said they are closely monitoring the haze.

A heavy downpour brought clearer skies over the affluent city-state Tuesday, but a strong smell of burning wood and foliage remained in the air. Businesses and schools were operating normally.

- 'Firm legal action' -

Smog-belching blazes, an annual problem in Southeast Asia during the dry season, have intensified in Sumatra and the Indonesian part of Borneo island in the past two weeks, sending a cloud of acrid haze across the region.

The illegal fires are started, often by local farmers and landowners, to clear land to make way for palm oil and pulp and paper plantations, and Indonesia has failed to halt the practice despite years of pressure from its neighbours.

More than half of Malaysia's 52 pollutant monitoring stations around the country registered "unhealthy" air on Tuesday.

After announcing late Monday that more troops would be sent to Sumatra, Widodo said he had ordered law enforcement agencies to take action against "parties responsible for the forest fires".

"I want to stress that very firm legal action will be taken," said the president, who is currently on a trip to the Middle East.

Around 100 people and 15 companies are being investigated over the blazes, according to the disaster agency.

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 attempts to find a regional solution have moved slowly.

Singapore said this week Indonesia had agreed to share the names of companies suspected of causing the fires. Indonesia has previously faced criticism for failing to hand over such information, which could be used to prosecute law-breakers.

Malaysian Environment Minister Wan Junaidi Tuanku Jaafar told AFP he was worried as the haze "affects the health of our people" and said he planned to meet his Indonesian counterpart, Siti Nurbaya Bakar, later this month to discuss the issue.

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