Fast-food deliveries have been cancelled, the army has suspended field training and even Singapore's top marathon runner has retreated as residents try to protect themselves from the smog that has descended on the city-state.

In Singapore's worst environmental crisis in more than a decade, the skyscrapers lining the Marina Bay financial district were shrouded by thick smoke Thursday as raging forest fires in neighbouring Indonesia's Sumatra island pushed air pollution levels to an all-time high.

Marathon runner Mok Ying Ren said the haze had forced him to run indoors on a gym treadmill as "it is just too crazy to run outdoors in these conditions".

"I tried running with a mask on, but after 45 minutes it is too sweaty and uncomfortable," said the 25-year-old doctor, who clocks 100 kilometres (62 miles) a week as part of a gruelling training programme to qualify for the 2016 Olympics.

Singapore's army on Wednesday night said it was suspending all field training "to ensure the well-being and safety of our soldiers".

Even a comforting takeaway has become harder to find as fast-food giants McDonald's, KFC and Pizza Hut have suspended deliveries due to safety concerns for their motorcycle-riding delivery staff.

Hunched commuters wore masks or covered their mouths as they walked home in the evening smog on Thursday, with major drug stores telling AFP they had temporarily run out of masks and refusing to accept advance orders.

"They're sold out. This morning some came in but after one hour they are gone already, over 200 packs," a saleswoman at a Guardian pharmacy in downtown Singapore told AFP by telephone.

"We can't take any reservations. It's on a first come, first served basis," she added.

The acrid smell of burning wood lingered everywhere, including inside air-conditioned metro trains, and cars were covered with a thin film of grey ash while the famed tree-lined parks and promenades were almost deserted.

While some companies gave their employees the day off on Thursday, life had to go on for many working people in the densely populated city-state, known for its obsessive cleanliness. Schools are currently on summer holiday.

But many Singaporeans expressed chagrin online, with some speculating that the government was manipulating the official pollutant standards index to downplay the severity of the crisis.

Prime Minister Lee Hsien Loong, who said the haze problem could persist for weeks due to the dry season, dismissed the rumours in a press conference on Thursday.

"We don't play those kinds of games in Singapore," he said.

Some found ways to make light of the situation.

One couple took wedding photographs against the usually spectacular backdrop of the Marina Bay Sands casino's triple hotel towers, Singapore's de facto national icon, which have become looming shadows in the smog.

Visitor numbers have dipped slightly at the popular Singapore Zoo but its exotic inhabitants are coping with the haze so far, a press statement said.

"At present, we are not facing any pressing health issues with the animals in our collection as a result of the haze. The animals are exhibiting normal behaviour with no noticeable adverse reactions towards the air pollution."

Southeast Asia's worst haze crisis took place in 1997-1998, causing widespread health problems and costing the regional economy billions of dollars as a result of business and air transport disruptions that lasted for weeks.

The last major haze outbreak in the region was in 2006.