Singaporeans reduced military training exercises, kept cough-stricken children indoors and wore protective masks to work after a smoky haze triggered by forest fires in neighbouring Indonesia caused air pollution to hit its worst level in nearly 16 years.

Singapore‘s main measurement of air quality has hovered at the unhealthy classification as smoke from blazes on Sumatra island drifted across the sea and cast a grey pall over the city-state’s skyscrapers.

The readings on the pollutant standards index were mostly between 104 and 123 on Tuesday, within the unhealthy range of between 101 and 200. A peak reading of 155 on Monday night was the highest since late 1997, when officials reported a 226 reading.

Smoke haze is an annual problem for Singapore and its northern neighbour, Malaysia, often beginning midyear, when farmers in Indonesia seek to clear land cheaply by starting fires. It sometimes causes diplomatic strains as Malaysia and Singapore urge Indonesia to do more to prevent illegal burning.

Malaysia has been only lightly affected so far this year, with pollution readings in Kuala Lumpur, its largest city, not breaching the unhealthy mark. Indonesia has said part of the current problem is caused by peat blazes that firefighters are struggling to extinguish during hot, dry weather.

The forest ministry spokesman, Sumarto Suharno, said the government was continuing to educate farmers about alternatives to traditional slash-and-burn agriculture. “We have been able to reduce the regional haze problem significantly for years with help from local communities and will continue to undertake all efforts to prevent it from spreading,” he said.

A boat sails along Marina bay, Singapore. Photograph: Roslan Rahman/AFP/GettyIn Singapore, the defence ministry spokesman, Kenneth Liow, said the armed forces had reduced physical and outdoor training accordingly after the pollution index exceeded 100.

Landscaper Hedrick Kwan blamed the haze after his two young sons started coughing as an acrid smell filled the air. “We usually leave the windows open but now we can’t do that because of all the dust and smoke,” he said.

The National Environment Agency has advised Singaporeans, especially the elderly, children and people with respiratory problems, to avoid prolonged exposure outdoors.

Melissa Cheah, a financial sector worker, said her office considered advising employees to wear protective masks.

Singapore’s environment minister, Vivian Balakrishnan, said his agency had offered to help Indonesia extinguish the forest fires.

Hospitals and clinics are expecting further smoke-related admissions next week.

“Based on what we have seen over the years, when the haze hits us, it takes about three to four days before we see all these additional patients coming in for medication,” said Sarani Ng Omar, a GP, who predicted a rise in asthma-related cases, nasal problems and eye inflammation.